Capsaicin

Capsaicin at a Glance

Capsaicin is the molecule that makes chillies hot. Chemically it is a capsaicinoid — a fatty-acid amide of vanillylamine — and it is made nowhere in nature except the genus Capsicum, principally in the placental tissue that holds the seeds ^{35,37Reference 35Al Othman ZA et al. · 2011Determination of capsaicin and dihydrocapsaicin in Capsicum fruit samples using high performance liquid chromatographyView study →Reference 37Stewart C Jr et al. · 2005The Pun1 gene for pungency in pepper encodes a putative acyltransferaseView study →}.

It occupies an unusual position among plant constituents. Most phytochemicals are studied through extracts and animal models; capsaicin has a named molecular receptor and an approved prescription drug. The receptor is TRPV1, cloned in 1997 using capsaicin as the probe and still routinely called the capsaicin receptor ^{1Reference 1Caterina MJ et al. · 1997The capsaicin receptor: a heat-activated ion channel in the pain pathwayView study →}. The drug is a high-concentration 8% topical patch, approved in the EU and USA for peripheral neuropathic pain ^{5,12Reference 5Anand P · 2011Topical capsaicin for pain management: therapeutic potential and mechanisms of action of the new high-concentration capsaicin 8% patchView study →Reference 12Burness CB · 2016ReviewCapsaicin 8% patch: a review in peripheral neuropathic painView study →}.

The honest headline is that capsaicin’s evidence splits hard by route, and the two halves should never be blended. The topical story is a genuine, regulator-assessed medicine whose effect size is real but modest ^{9Reference 9Derry S et al. · 2017Meta-analysisTopical capsaicin (high concentration) for chronic neuropathic pain in adultsView study →}. The dietary story — thermogenesis, appetite, cardiometabolic and mortality epidemiology — is a separate exposure at a thousandth of the concentration, with consistent but small measured effects ^{21,22,23Reference 21Ludy MJ et al. · 2012Meta-analysisThe effects of capsaicin and capsiate on energy balance: critical review and meta-analyses of studies in humansView study →Reference 22Whiting S et al. · 2012Systematic reviewCapsaicinoids and capsinoidsView study →Reference 23Lv J et al. · 2015ObservationalConsumption of spicy foods and total and cause specific mortality: population based cohort studyView study →}. And the safety story is dominated not by systemic toxicity but by the fact that this same molecule is the active ingredient of pepper spray ^{40Reference 40Busker RW · 1998Toxicologic evaluation of pepper spray as a possible weapon for the Dutch police force: risk assessment and efficacyView study →}.

Established Medical Uses

Capsaicin is one of the small number of plant constituents that is itself a licensed drug rather than a supplement ingredient. Its established human uses are all topical or topically applied to a mucous membrane:

• High-concentration (8%) cutaneous patch — approved in the EU and USA for peripheral neuropathic pain, including post-herpetic neuralgia and painful diabetic peripheral neuropathy of the feet, given as a single supervised 30–60 minute application repeatable at about 12-week intervals ^{5,12Reference 5Anand P · 2011Topical capsaicin for pain management: therapeutic potential and mechanisms of action of the new high-concentration capsaicin 8% patchView study →Reference 12Burness CB · 2016ReviewCapsaicin 8% patch: a review in peripheral neuropathic painView study →}.
• Low-concentration creams (0.025–0.075%) — long-standing over-the-counter use for osteoarthritis, musculoskeletal pain and diabetic neuropathy, applied three to four times a day ^{14,15,16Reference 14Mason L et al. · 2004Systematic reviewSystematic review of topical capsaicin for the treatment of chronic painView study →Reference 15Zhang WY · 1994Meta-analysisThe effectiveness of topically applied capsaicinView study →Reference 16Deal CL et al. · 1991RCTTreatment of arthritis with topical capsaicin: a double-blind trialView study →}.
• Intranasal capsaicin — studied as a treatment for non-allergic (idiopathic) rhinitis ^{26Reference 26Gevorgyan A et al. · 2015Systematic reviewCapsaicin for non-allergic rhinitisView study →}.
• Laboratory and diagnostic use — inhaled capsaicin is the standard provocation agent for measuring cough-reflex sensitivity ^{44Reference 44Koskela HO et al. · 2021ReviewUtility of cough provocation tests in chronic cough and respiratory diseases: a comprehensive review and introduction of new reference rangesView study →}.

There is no established oral medicinal use of isolated capsaicin. The oral literature is nutritional and epidemiological, not therapeutic.

TRPV1 — The Capsaicin Receptor

Capsaicin’s pharmacology is unusually clean for a plant molecule because it acts, at the doses that matter, on essentially one target.

TRPV1 (transient receptor potential vanilloid 1) was cloned in 1997 by expression-screening a sensory-neuron library for capsaicin responsiveness, and was described in that first paper as a heat-activated ion channel in the pain pathway ^{1Reference 1Caterina MJ et al. · 1997The capsaicin receptor: a heat-activated ion channel in the pain pathwayView study →}. It is a non-selective cation channel, highly permeable to calcium, expressed on small-diameter nociceptive sensory neurons — the C and Aδ fibres that carry burning pain.

The point that matters, and that is usually stated loosely, is that TRPV1 is a polymodal nociceptor. The cloned channel is gated by capsaicin, by noxious heat above roughly 43 °C, and by acid (extracellular protons), and those stimuli act cooperatively on the same protein ^{1,2Reference 1Caterina MJ et al. · 1997The capsaicin receptor: a heat-activated ion channel in the pain pathwayView study →Reference 2Tominaga M et al. · 1998The cloned capsaicin receptor integrates multiple pain-producing stimuliView study →}. This is why chilli feels hot in a way that is not a metaphor and not a linguistic accident: capsaicin opens the identical channel that tissue-damaging heat opens, and the nervous system has no way to distinguish the two signals. It is also why acidosis in inflamed tissue sensitises the same fibres, and why chilli burn is worsened by heat and by acid.

Structural work has since resolved how the molecule binds. Capsaicin’s vanillyl head group inserts into a pocket between the S3–S4 helices and the S4–S5 linker of a TRPV1 subunit, adopting a “tail-up, head-down” configuration; the interaction is anchored by hydrogen bonding of the vanillyl and amide groups, and the channel’s capsaicin sensitivity can be transferred between species by swapping just a few pocket residues ^{3,4Reference 3Yang F · 2017Understand spiciness: mechanism of TRPV1 channel activation by capsaicinView study →Reference 4Yang F et al. · 2015Structural mechanism underlying capsaicin binding and activation of the TRPV1 ion channelView study →}. That structural specificity is why the rodent-vs-avian difference exists at all — see Its Role in Plants below.

Defunctionalisation, Not Substance-P Depletion

The therapeutic mechanism of high-concentration topical capsaicin is the single most commonly mis-stated fact about this molecule, and it is worth getting exactly right.

The older account — that capsaicin works by depleting substance P from nociceptors — is largely superseded. Experimental and clinical work shows that substance-P depletion is only a correlate of capsaicin treatment and has little if any causative role in the pain relief ^{5Reference 5Anand P · 2011Topical capsaicin for pain management: therapeutic potential and mechanisms of action of the new high-concentration capsaicin 8% patchView study →}.

What actually happens is a process best described as defunctionalisation of nociceptor fibres. Sustained TRPV1 activation drives a large, prolonged calcium influx into the nerve terminal, which produces several overlapping effects: temporary loss of membrane potential; loss of the ability to transport neurotrophic factors, altering the neuron’s phenotype; and — the load-bearing one — reversible retraction of the epidermal and dermal nerve-fibre terminals ^{5Reference 5Anand P · 2011Topical capsaicin for pain management: therapeutic potential and mechanisms of action of the new high-concentration capsaicin 8% patchView study →}. Mechanistically, the terminal ablation is calcium- and calpain-dependent: the calcium load activates calpain proteases that dismantle the axonal cytoskeleton of the TRPV1-expressing terminal, while the cell body survives ^{8Reference 8Wang S et al. · 2017Ca²⁺ and calpain mediate capsaicin-induced ablation of axonal terminals expressing transient receptor potential vanilloid 1View study →}.

The human evidence for this is direct and quantitative. Skin biopsies after a single 60-minute 8% patch application in healthy volunteers showed roughly an 80% reduction in epidermal nerve-fibre density at one week; regeneration was under way but incomplete at 12 weeks, and nearly complete (93%) at 24 weeks ^{7Reference 7Kennedy WR et al. · 2010RCTA randomized, controlled, open-label study of the long-term effects of NGX-4010, a high-concentration capsaicin patch, on epidermal nerve fiber density and sensory function in healthy volunteersView study →}. Earlier work with repeated topical capsaicin showed the same thing — parallel loss of epidermal nerve fibres and of pain sensation, both recovering together ^{6Reference 6Nolano M et al. · 1999Topical capsaicin in humans: parallel loss of epidermal nerve fibers and pain sensationView study →}.

Two practical consequences follow, and they explain the drug’s whole clinical shape:

1. One brief application can give weeks of relief. The effect does not depend on the drug remaining in the tissue — it depends on nerve terminals having withdrawn and needing to regrow. Systemic capsaicin is essentially gone within hours ^{17Reference 17Babbar S et al. · 2009Pharmacokinetic analysis of capsaicin after topical administration of a high-concentration capsaicin patch to patients with peripheral neuropathic painView study →}.
2. The initial application burns, and it must. The therapeutic mechanism is intense TRPV1 activation. There is no version of this treatment that does not first hurt, which is why the patch is applied under supervision, usually after local anaesthetic or with cooling ^{5,9Reference 5Anand P · 2011Topical capsaicin for pain management: therapeutic potential and mechanisms of action of the new high-concentration capsaicin 8% patchView study →Reference 9Derry S et al. · 2017Meta-analysisTopical capsaicin (high concentration) for chronic neuropathic pain in adultsView study →}.

The 8% Patch

The prescription product is a cutaneous patch delivering capsaicin at 8% w/w (640 µg/cm², 179 mg per patch) — about 100 times the concentration of conventional OTC creams ^{9,17Reference 9Derry S et al. · 2017Meta-analysisTopical capsaicin (high concentration) for chronic neuropathic pain in adultsView study →Reference 17Babbar S et al. · 2009Pharmacokinetic analysis of capsaicin after topical administration of a high-concentration capsaicin patch to patients with peripheral neuropathic painView study →}. It is applied once, for 30 minutes on the feet or 60 minutes elsewhere, and the benefit is expected to last around 12 weeks before another application ^{9Reference 9Derry S et al. · 2017Meta-analysisTopical capsaicin (high concentration) for chronic neuropathic pain in adultsView study →}.

Regulatory status. Approved in both the EU and USA for peripheral neuropathic pain; the initial indication was post-herpetic neuralgia, later extended to painful diabetic peripheral neuropathy of the feet ^{5,11,12Reference 5Anand P · 2011Topical capsaicin for pain management: therapeutic potential and mechanisms of action of the new high-concentration capsaicin 8% patchView study →Reference 11Simpson DM et al. · 2017RCTCapsaicin 8% patch in painful diabetic peripheral neuropathy: a randomized, double-blind, placebo-controlled studyView study →Reference 12Burness CB · 2016ReviewCapsaicin 8% patch: a review in peripheral neuropathic painView study →}.

What the trials show. The pivotal randomised, double-blind studies used a 0.04% capsaicin “active placebo” to preserve blinding — an important design detail, because it means the comparator was not inert. Backonja’s post-herpetic neuralgia trial found a significantly greater mean reduction in pain over weeks 2–12 with the 8% patch than with the low-dose control ^{10Reference 10Backonja M et al. · 2008RCTNGX-4010, a high-concentration capsaicin patch, for the treatment of postherpetic neuralgia: a randomised, double-blind studyView study →}; Simpson’s diabetic-neuropathy trial reported the same direction of effect ^{11Reference 11Simpson DM et al. · 2017RCTCapsaicin 8% patch in painful diabetic peripheral neuropathy: a randomized, double-blind, placebo-controlled studyView study →}.

What the meta-analysis shows, honestly. The 2017 Cochrane review pooled eight studies and 2,488 participants ^{9Reference 9Derry S et al. · 2017Meta-analysisTopical capsaicin (high concentration) for chronic neuropathic pain in adultsView study →}. For post-herpetic neuralgia, about 10% more participants reported being much or very much improved at 8 and 12 weeks with the 8% patch than with active placebo, giving a number needed to treat of 8.8 (95% CI 5.3 to 26); NNTs for a 30% or 50% pain reduction fell between 10 and 12. For painful HIV neuropathy the NNT was about 11. Serious adverse events were no more common with active treatment (3.5%) than control (3.2%). The review graded the evidence moderate to very low quality and concluded that the additional proportion who benefit over control is not large — but that those who do get high levels of relief also improve on sleep, fatigue, depression and quality of life ^{9Reference 9Derry S et al. · 2017Meta-analysisTopical capsaicin (high concentration) for chronic neuropathic pain in adultsView study →}.

That is the correct posture: a real, approved, mechanistically well-understood treatment with a modest effect size — comparable to, not better than, other chronic-pain therapies ^{9Reference 9Derry S et al. · 2017Meta-analysisTopical capsaicin (high concentration) for chronic neuropathic pain in adultsView study →}.

The low-concentration creams are a much weaker evidence base. The parallel Cochrane review of <1% capsaicin creams found only six small studies, all published before 1996, with inconsistent results and insufficient data to draw conclusions about efficacy in neuropathic pain — while local skin reactions were common enough to give a number needed to harm of 2.5 ^{13Reference 13Derry S · 2012Meta-analysisTopical capsaicin (low concentration) for chronic neuropathic pain in adultsView study →}. Older pooled analyses of 0.075% cream in neuropathic pain (NNT 5.7) and 0.025% cream in musculoskeletal pain (NNT 8.1) look more favourable, but the reviewers themselves flagged that blinding is nearly impossible when one arm burns and the other does not ^{14,15Reference 14Mason L et al. · 2004Systematic reviewSystematic review of topical capsaicin for the treatment of chronic painView study →Reference 15Zhang WY · 1994Meta-analysisThe effectiveness of topically applied capsaicinView study →}. The patch and the cream should not be spoken of as one treatment.

Scoville Units & Standardisation

The Scoville scale is older than the chemistry it now proxies for. In 1912 the pharmacist Wilbur Scoville published an organoleptic dilution assay: an alcohol extract of ground chilli was progressively diluted with sweetened water until a panel of tasters could no longer detect heat, and the dilution factor became the pepper’s rating in Scoville Heat Units ^{49Reference 49Scoville WL · 1912Note on CapsicumsView study →}. It was a taste test with a number attached.

Modern practice measures capsaicinoids directly by HPLC and converts. The conversion used in analytical work is about 16 SHU per part-per-million of capsaicin: in one market survey, hot chillies at 4,249 µg/g capsaicin were rated 67,985 SHU, red chillies at 309 µg/g were rated 4,949 SHU, and green sweet peppers at 1.0 µg/g came out at 16 SHU ^{35Reference 35Al Othman ZA et al. · 2011Determination of capsaicin and dihydrocapsaicin in Capsicum fruit samples using high performance liquid chromatographyView study →}. On that arithmetic pure capsaicin sits at roughly 16 million SHU — the ceiling the scale is anchored to.

Two caveats belong with any Scoville figure:

• SHU is a pungency number, not a capsaicin assay. Different capsaicinoids have different pungency per milligram — pungency is a sensory property of a whole chemical class, of which capsaicin is only the best-known member ^{34Reference 34Nilius B · 2013Spices: the savory and beneficial science of pungencyView study →} — so a rating derived from capsaicin alone will not capture a fruit whose heat is carried by dihydrocapsaicin or nordihydrocapsaicin.
• Capsaicin is not always the dominant capsaicinoid. Capsaicin and dihydrocapsaicin together make up roughly 90% of total capsaicinoids, with capsaicin usually about 71% of the total in pungent varieties ^{35Reference 35Al Othman ZA et al. · 2011Determination of capsaicin and dihydrocapsaicin in Capsicum fruit samples using high performance liquid chromatographyView study →} — but this is chemotype-dependent, and in one HPLC survey of eight C. annuum varieties dihydrocapsaicin exceeded capsaicin in every one ^{36Reference 36González-Zamora A et al. · 2013Characterization of different Capsicum varieties by evaluation of their capsaicinoids content by high performance liquid chromatography, determination of pungency and effect of high temperatureView study →}. The same reversal shows in the market-survey data above for hot and green chillies ^{35Reference 35Al Othman ZA et al. · 2011Determination of capsaicin and dihydrocapsaicin in Capsicum fruit samples using high performance liquid chromatographyView study →}.

For therapeutic purposes none of this matters, because the medicine is purified capsaicin at a stated concentration, not a fruit at a stated Scoville rating. That is the whole reason the clinical literature reports milligrams of isolate and not grams of chilli.

Dietary Capsaicin vs. Topical Capsaicin

These are different exposures and should be reasoned about separately.

Aspect	Topical (patch / cream)	Dietary (chilli / capsules)
Concentration	8% patch; 0.025–0.075% creams 9,13Reference 9Derry S et al. · 2017Meta-analysisTopical capsaicin (high concentration) for chronic neuropathic pain in adultsView study →Reference 13Derry S · 2012Meta-analysisTopical capsaicin (low concentration) for chronic neuropathic pain in adultsView study →	~0.4% of a hot fresh chilli at the top end 35Reference 35Al Othman ZA et al. · 2011Determination of capsaicin and dihydrocapsaicin in Capsicum fruit samples using high performance liquid chromatographyView study →
Typical exposure	179 mg once, to a defined skin area 9Reference 9Derry S et al. · 2017Meta-analysisTopical capsaicin (high concentration) for chronic neuropathic pain in adultsView study →	~7.6 mg/person/day in a chilli-eating population 35Reference 35Al Othman ZA et al. · 2011Determination of capsaicin and dihydrocapsaicin in Capsicum fruit samples using high performance liquid chromatographyView study →
Target tissue	Cutaneous nociceptor terminals	Oral, gastric and intestinal TRPV1; systemic after absorption
Mechanism of benefit	Nociceptor defunctionalisation 5Reference 5Anand P · 2011Topical capsaicin for pain management: therapeutic potential and mechanisms of action of the new high-concentration capsaicin 8% patchView study →	Sympathetic/thermogenic activation, TRPV1 in gut and vasculature 21,25Reference 21Ludy MJ et al. · 2012Meta-analysisThe effects of capsaicin and capsiate on energy balance: critical review and meta-analyses of studies in humansView study →Reference 25Yang D et al. · 2010Activation of TRPV1 by dietary capsaicin improves endothelium-dependent vasorelaxation and prevents hypertensionView study →
Evidence tier	Multiple RCTs, Cochrane review, regulatory approval 9Reference 9Derry S et al. · 2017Meta-analysisTopical capsaicin (high concentration) for chronic neuropathic pain in adultsView study →	Human meta-analyses of small crossover studies + observational cohorts 21,22,23Reference 21Ludy MJ et al. · 2012Meta-analysisThe effects of capsaicin and capsiate on energy balance: critical review and meta-analyses of studies in humansView study →Reference 22Whiting S et al. · 2012Systematic reviewCapsaicinoids and capsinoidsView study →Reference 23Lv J et al. · 2015ObservationalConsumption of spicy foods and total and cause specific mortality: population based cohort studyView study →
Effect size	Modest (NNT ~9–12) 9Reference 9Derry S et al. · 2017Meta-analysisTopical capsaicin (high concentration) for chronic neuropathic pain in adultsView study →	Small (~50 kcal/day energy expenditure) 22Reference 22Whiting S et al. · 2012Systematic reviewCapsaicinoids and capsinoidsView study →

A finding in one column is not evidence for the other. The patch trials say nothing about whether eating chilli helps pain; the mortality cohorts say nothing about neuropathy.

What It's Used For

Capsaicin’s research base is stratified by route and by evidence tier more sharply than almost any other plant molecule here — from regulator-assessed randomised trials at the top to in-vitro cancer work at the bottom.

Evidence by IndicationStrength of support · Beta

Support is an experimental score — weighted by study type (human > animal > in vitro) and volume. A beta, not a validated metric. Note that the top row is an approved topical medicine and the middle rows are dietary exposures — the score cannot be read across routes.

01Neuropathic pain (high-concentration patch)Promising72%Multiple RCTs + Cochrane meta-analysis (8 studies, 2,488 participants); EU/US approval; direct nerve-fibre biopsy mechanism ^{5,7,9,10,11Reference 5Anand P · 2011Topical capsaicin for pain management: therapeutic potential and mechanisms of action of the new high-concentration capsaicin 8% patchView study →Reference 7Kennedy WR et al. · 2010RCTA randomized, controlled, open-label study of the long-term effects of NGX-4010, a high-concentration capsaicin patch, on epidermal nerve fiber density and sensory function in healthy volunteersView study →Reference 9Derry S et al. · 2017Meta-analysisTopical capsaicin (high concentration) for chronic neuropathic pain in adultsView study →Reference 10Backonja M et al. · 2008RCTNGX-4010, a high-concentration capsaicin patch, for the treatment of postherpetic neuralgia: a randomised, double-blind studyView study →Reference 11Simpson DM et al. · 2017RCTCapsaicin 8% patch in painful diabetic peripheral neuropathy: a randomized, double-blind, placebo-controlled studyView study →}

02Musculoskeletal & osteoarthritis pain (creams)Promising46%Pooled RCTs of 0.025–0.075% creams (NNT 5.7–8.1) but blinding compromised and the low-concentration Cochrane review is inconclusive ^{13,14,15,16Reference 13Derry S · 2012Meta-analysisTopical capsaicin (low concentration) for chronic neuropathic pain in adultsView study →Reference 14Mason L et al. · 2004Systematic reviewSystematic review of topical capsaicin for the treatment of chronic painView study →Reference 15Zhang WY · 1994Meta-analysisThe effectiveness of topically applied capsaicinView study →Reference 16Deal CL et al. · 1991RCTTreatment of arthritis with topical capsaicin: a double-blind trialView study →}

03Thermogenesis & body-weight managementShaky42%Three human meta-analyses/systematic reviews; consistent direction, small magnitude ^{20,21,22Reference 20Zsiborás C et al. · 2018Meta-analysisCapsaicin and capsiate could be appropriate agents for treatment of obesity: a meta-analysis of human studiesView study →Reference 21Ludy MJ et al. · 2012Meta-analysisThe effects of capsaicin and capsiate on energy balance: critical review and meta-analyses of studies in humansView study →Reference 22Whiting S et al. · 2012Systematic reviewCapsaicinoids and capsinoidsView study →}

04Non-allergic rhinitis (intranasal)Shaky34%Cochrane review of small intranasal RCTs; low-quality evidence ^{26Reference 26Gevorgyan A et al. · 2015Systematic reviewCapsaicin for non-allergic rhinitisView study →}

05Cardiometabolic & mortality epidemiologyShaky32%Two large prospective cohorts (n≈487,000 and n≈16,000) + rodent TRPV1 vascular mechanism; observational ^{23,24,25Reference 23Lv J et al. · 2015ObservationalConsumption of spicy foods and total and cause specific mortality: population based cohort studyView study →Reference 24Chopan M · 2017ObservationalThe association of hot red chili pepper consumption and mortality: a large population-based cohort studyView study →Reference 25Yang D et al. · 2010Activation of TRPV1 by dietary capsaicin improves endothelium-dependent vasorelaxation and prevents hypertensionView study →}

06Gastric protection & functional GI symptomsShaky28%Case-control and small trials that reverse the older ulcer-causation claim; contested literature ^{27,28,29Reference 27Satyanarayana MN · 2006Capsaicin and gastric ulcersView study →Reference 28Kang JY et al. · 1995Chili — protective factor against peptic ulcer? Digestive Diseases and Sciences. https://pubmed.ncbi.nlm.nih.gov/7895548/View study →Reference 29Bortolotti M · 2011Clinical trialEffect of red pepper on symptoms of irritable bowel syndrome: preliminary studyView study →}

07Cannabinoid hyperemesis (topical)Shaky22%Case series only; no randomised trial ^{30,31Reference 30Dezieck L et al. · 2017Case reportResolution of cannabis hyperemesis syndrome with topical capsaicin in the emergency department: a case seriesView study →Reference 31Graham J et al. · 2017Case reportCapsaicin cream for treatment of cannabinoid hyperemesis syndrome in adolescents: a case seriesView study →}

08AnticancerShaky16%In vitro and animal only, and the same literature documents a tumour-promoter signal ^{32,33Reference 32Bode AM · 2011The two faces of capsaicinView study →Reference 33Fattori V et al. · 2016Capsaicin: current understanding of its mechanisms and therapy of pain and other pre-clinical and clinical usesView study →}

Mechanisms

The whole page collapses into one target. Almost everything capsaicin does — the burn, the analgesia, the cough, the thermogenesis, the pepper-spray effect — is downstream of TRPV1 activation in a different tissue. The therapeutic trick is not blocking that receptor but over-activating it until the nerve ending withdraws.

Pharmacokinetics

Topical, the route that matters clinically, is defined by how little gets in. Population PK analysis of 173 patients receiving a single 60- or 90-minute 8% patch application found quantifiable plasma capsaicin in only 31% of post-herpetic-neuralgia patients, 7% of HIV-neuropathy patients and 3% of diabetic-neuropathy patients; the maximum concentration observed in any patient was 17.8 ng/mL, mean Cmax after a 60-minute application was 1.86 ng/mL, and the population elimination half-life was 1.64 hours ^{17Reference 17Babbar S et al. · 2009Pharmacokinetic analysis of capsaicin after topical administration of a high-concentration capsaicin patch to patients with peripheral neuropathic painView study →}. Treatment on the feet produced far lower exposure than on the trunk. The authors’ conclusion is the key safety fact of the whole page: this exposure is too low and too brief to produce systemic effects or drug–drug interactions ^{5,17Reference 5Anand P · 2011Topical capsaicin for pain management: therapeutic potential and mechanisms of action of the new high-concentration capsaicin 8% patchView study →Reference 17Babbar S et al. · 2009Pharmacokinetic analysis of capsaicin after topical administration of a high-concentration capsaicin patch to patients with peripheral neuropathic painView study →}. The weeks of analgesia therefore cannot be a plasma-level phenomenon — it is nerve-terminal regrowth time ^{7Reference 7Kennedy WR et al. · 2010RCTA randomized, controlled, open-label study of the long-term effects of NGX-4010, a high-concentration capsaicin patch, on epidermal nerve fiber density and sensory function in healthy volunteersView study →}.

Orally, capsaicin is rapidly absorbed but heavily first-pass metabolised, and its systemic bioavailability is a well-recognised limitation on translating in-vitro findings ^{18Reference 18Rollyson WD et al. · 2014Bioavailability of capsaicin and its implications for drug deliveryView study →}. Metabolism is cytochrome-P450-dependent and broad: recombinant-enzyme work implicates CYP1A1, 1A2, 2B6, 2C8, 2C9, 2C19, 2D6, 2E1 and 3A4, producing aromatic and alkyl hydroxylation products plus a novel dehydrogenation pathway yielding macrocyclic, diene and imide metabolites; glutathione trapped several reactive electrophilic intermediates, and metabolism reduced cytotoxicity to lung and liver cells ^{19Reference 19Reilly CA et al. · 2003Metabolism of capsaicin by cytochrome P450 produces novel dehydrogenated metabolites and decreases cytotoxicity to lung and liver cellsView study →}. Two implications: the reactive-intermediate chemistry is the plausible basis for the tumour-promoter signal, and the extensive CYP involvement is a theoretical interaction surface for high-dose oral capsaicin — though it is not clinically characterised, and is irrelevant to the patch given its negligible systemic exposure ^{17,19Reference 17Babbar S et al. · 2009Pharmacokinetic analysis of capsaicin after topical administration of a high-concentration capsaicin patch to patients with peripheral neuropathic painView study →Reference 19Reilly CA et al. · 2003Metabolism of capsaicin by cytochrome P450 produces novel dehydrogenated metabolites and decreases cytotoxicity to lung and liver cellsView study →}.

Clinical Trials

Capsaicin’s clinical footprint is real but narrow, and almost entirely topical. The 8% patch has completed multiple pivotal randomised double-blind trials across post-herpetic neuralgia, painful HIV neuropathy and painful diabetic peripheral neuropathy, pooled in Cochrane reviews and assessed by EU and US regulators ^{9,10,11,12Reference 9Derry S et al. · 2017Meta-analysisTopical capsaicin (high concentration) for chronic neuropathic pain in adultsView study →Reference 10Backonja M et al. · 2008RCTNGX-4010, a high-concentration capsaicin patch, for the treatment of postherpetic neuralgia: a randomised, double-blind studyView study →Reference 11Simpson DM et al. · 2017RCTCapsaicin 8% patch in painful diabetic peripheral neuropathy: a randomized, double-blind, placebo-controlled studyView study →Reference 12Burness CB · 2016ReviewCapsaicin 8% patch: a review in peripheral neuropathic painView study →}. Low-concentration creams have an older, smaller and methodologically weaker trial base ^{13,14,15Reference 13Derry S · 2012Meta-analysisTopical capsaicin (low concentration) for chronic neuropathic pain in adultsView study →Reference 14Mason L et al. · 2004Systematic reviewSystematic review of topical capsaicin for the treatment of chronic painView study →Reference 15Zhang WY · 1994Meta-analysisThe effectiveness of topically applied capsaicinView study →}. Dietary capsaicin has been tested in small human crossover thermogenesis studies pooled into meta-analyses ^{20,21,22Reference 20Zsiborás C et al. · 2018Meta-analysisCapsaicin and capsiate could be appropriate agents for treatment of obesity: a meta-analysis of human studiesView study →Reference 21Ludy MJ et al. · 2012Meta-analysisThe effects of capsaicin and capsiate on energy balance: critical review and meta-analyses of studies in humansView study →Reference 22Whiting S et al. · 2012Systematic reviewCapsaicinoids and capsinoidsView study →}, and intranasal capsaicin in small rhinitis RCTs ^{26Reference 26Gevorgyan A et al. · 2015Systematic reviewCapsaicin for non-allergic rhinitisView study →}. There are no randomised trials of oral capsaicin for pain, and none for cancer.

Last checked: July 2026.

Isolate vs. Plant Studies

Capsaicin inverts the usual compound-page caveat, and it is one of the cleanest cases here.

The clinical evidence is the isolate — all of it. The 8% patch is purified capsaicin at 640 µg/cm² ^{17Reference 17Babbar S et al. · 2009Pharmacokinetic analysis of capsaicin after topical administration of a high-concentration capsaicin patch to patients with peripheral neuropathic painView study →}; the creams are purified capsaicin at 0.025–0.075% ^{14,15,16Reference 14Mason L et al. · 2004Systematic reviewSystematic review of topical capsaicin for the treatment of chronic painView study →Reference 15Zhang WY · 1994Meta-analysisThe effectiveness of topically applied capsaicinView study →Reference 16Deal CL et al. · 1991RCTTreatment of arthritis with topical capsaicin: a double-blind trialView study →}; the intranasal studies use capsaicin solutions ^{26Reference 26Gevorgyan A et al. · 2015Systematic reviewCapsaicin for non-allergic rhinitisView study →}; the thermogenesis trials use purified capsaicin, capsaicinoid extracts or capsiate ^{20,21Reference 20Zsiborás C et al. · 2018Meta-analysisCapsaicin and capsiate could be appropriate agents for treatment of obesity: a meta-analysis of human studiesView study →Reference 21Ludy MJ et al. · 2012Meta-analysisThe effects of capsaicin and capsiate on energy balance: critical review and meta-analyses of studies in humansView study →}. No randomised trial of a cayenne fruit preparation underwrites any of the therapeutic claims here. For a plant molecule, that is a genuine strength — the dose is known.

The whole-plant evidence is epidemiological, not therapeutic. The mortality cohorts measured self-reported spicy-food frequency, with no capsaicin quantification at all ^{23,24Reference 23Lv J et al. · 2015ObservationalConsumption of spicy foods and total and cause specific mortality: population based cohort studyView study →Reference 24Chopan M · 2017ObservationalThe association of hot red chili pepper consumption and mortality: a large population-based cohort studyView study →}, and the ulcer case-control study counted chilli use, not milligrams ^{28Reference 28Kang JY et al. · 1995Chili — protective factor against peptic ulcer? Digestive Diseases and Sciences. https://pubmed.ncbi.nlm.nih.gov/7895548/View study →}. Whole-fruit exposure is also intrinsically unstandardisable: capsaicin content varies from undetectable to ~4,200 mg/kg between market samples of the same genus, and even the capsaicin-to-dihydrocapsaicin ratio flips between cultivars ^{35,36Reference 35Al Othman ZA et al. · 2011Determination of capsaicin and dihydrocapsaicin in Capsicum fruit samples using high performance liquid chromatographyView study →Reference 36González-Zamora A et al. · 2013Characterization of different Capsicum varieties by evaluation of their capsaicinoids content by high performance liquid chromatography, determination of pungency and effect of high temperatureView study →}.

The direction of transfer only runs one way. Because cayenne contains capsaicin, the patch trials are sometimes cited as evidence for cayenne. They are not. A 60-minute 8% application delivers 179 mg of purified molecule to a defined skin area under supervision; no culinary or herbal use of the fruit approaches that exposure or that control.

Its Role in Plants

Capsaicin is made in the placental tissue of the Capsicum fruit — the pale septum the seeds attach to, not the seeds themselves and not primarily the flesh. Its biosynthesis joins two pathways: vanillylamine, from the phenylpropanoid route, is condensed with 8-methyl-6-nonenoyl-CoA, from branched-chain fatty-acid synthesis. The condensing step is controlled by the Pun1 locus, which encodes a putative acyltransferase (AT3); a deletion in Pun1 is what makes bell peppers non-pungent ^{37Reference 37Stewart C Jr et al. · 2005The Pun1 gene for pungency in pepper encodes a putative acyltransferaseView study →}.

The ecological function is one of the more elegant stories in plant chemistry, and it turns on a receptor difference. Mammals have capsaicin-sensitive TRPV1; birds do not — avian TRPV1 lacks the residues that make the vanillyl pocket responsive ^{3Reference 3Yang F · 2017Understand spiciness: mechanism of TRPV1 channel activation by capsaicinView study →}. Mammals chew and grind chilli seeds, destroying them; birds swallow them whole and disperse them intact. Capsaicin is therefore a directed deterrent: a chemical that repels the seed predators while leaving the seed dispersers unaffected ^{38Reference 38Tewksbury JJ · 2001Seed dispersalView study →}.

A second, complementary pressure is microbial. In wild Capsicum chacoense, populations are polymorphic for capsaicinoid production, and the proportion of pungent plants tracks the local risk of Fusarium fungal seed mortality — capsaicinoids protect seeds from Fusarium, and hemipteran insect feeding is the entry route that lets the fungus in ^{39Reference 39Tewksbury JJ et al. · 2008Evolutionary ecology of pungency in wild chiliesView study →}. Pungency in wild chillies is best read as an anti-fungal defence that also happens to deter the wrong dispersers.

Prevalence in Nature

Capsaicin is taxonomically confined. Unlike most of the molecules here, it is not scattered across families — it is restricted to the genus Capsicum (Solanaceae), and within the genus its concentration is a cultivar-group trait rather than a species trait ^{35Reference 35Al Othman ZA et al. · 2011Determination of capsaicin and dihydrocapsaicin in Capsicum fruit samples using high performance liquid chromatographyView study →}. Sweet bell peppers, paprika, jalapeño and cayenne are all C. annuum; the first has essentially none and the last has a great deal.

Reported concentrations in fresh market fruit span more than three orders of magnitude: ~4,249 µg/g in hot chillies, 309 µg/g in red chillies, 139 µg/g in green chillies, and 1.0 µg/g in green sweet peppers (red and yellow sweet peppers were below detection) ^{35Reference 35Al Othman ZA et al. · 2011Determination of capsaicin and dihydrocapsaicin in Capsicum fruit samples using high performance liquid chromatographyView study →}. On a dry-weight basis, total capsaicinoids ranged from not detectable in bell pepper to 31.84 mg/g (~3.2%) in chiltepín across eight C. annuum varieties, and growing temperature shifted the content in both directions depending on cultivar ^{36Reference 36González-Zamora A et al. · 2013Characterization of different Capsicum varieties by evaluation of their capsaicinoids content by high performance liquid chromatography, determination of pungency and effect of high temperatureView study →}.

Capsaicin never occurs alone. It sits in a family of structurally related capsaicinoids — dihydrocapsaicin, nordihydrocapsaicin, homocapsaicin and homodihydrocapsaicin — of which capsaicin and dihydrocapsaicin together account for about 90% ^{35Reference 35Al Othman ZA et al. · 2011Determination of capsaicin and dihydrocapsaicin in Capsicum fruit samples using high performance liquid chromatographyView study →}. Ripe fruit also carries the red keto-carotenoids capsanthin and capsorubin plus beta-carotene, which are pigments and antioxidants with no bearing on pungency.

Major Natural Sources

Source	Plant part	Reported capsaicin content
Hot chilli (Capsicum spp., pungent market samples)	fresh fruit	~4,249 µg/g (≈68,000 SHU) 35Reference 35Al Othman ZA et al. · 2011Determination of capsaicin and dihydrocapsaicin in Capsicum fruit samples using high performance liquid chromatographyView study →
Red chilli (Capsicum spp.)	fresh fruit	~309 µg/g (≈4,900 SHU) 35Reference 35Al Othman ZA et al. · 2011Determination of capsaicin and dihydrocapsaicin in Capsicum fruit samples using high performance liquid chromatographyView study →
Green chilli (Capsicum spp.)	fresh fruit	~139 µg/g (≈2,200 SHU) 35Reference 35Al Othman ZA et al. · 2011Determination of capsaicin and dihydrocapsaicin in Capsicum fruit samples using high performance liquid chromatographyView study →
Chiltepín (C. annuum, wild)	dried fruit	total capsaicinoids ~31.8 mg/g dry wt 36Reference 36González-Zamora A et al. · 2013Characterization of different Capsicum varieties by evaluation of their capsaicinoids content by high performance liquid chromatography, determination of pungency and effect of high temperatureView study →
Green sweet pepper (C. annuum)	fresh fruit	~1.0 µg/g (≈16 SHU) 35Reference 35Al Othman ZA et al. · 2011Determination of capsaicin and dihydrocapsaicin in Capsicum fruit samples using high performance liquid chromatographyView study →
Red / yellow sweet pepper (C. annuum)	fresh fruit	below detection 35Reference 35Al Othman ZA et al. · 2011Determination of capsaicin and dihydrocapsaicin in Capsicum fruit samples using high performance liquid chromatographyView study →

Figures are fresh weight except where dry weight is stated, and they are single-study market or field samples — cultivar, ripeness, growing temperature and even fruit-to-fruit variation on the same plant shift them substantially ^{35,36Reference 35Al Othman ZA et al. · 2011Determination of capsaicin and dihydrocapsaicin in Capsicum fruit samples using high performance liquid chromatographyView study →Reference 36González-Zamora A et al. · 2013Characterization of different Capsicum varieties by evaluation of their capsaicinoids content by high performance liquid chromatography, determination of pungency and effect of high temperatureView study →}.

Discovery & Synthesis

Capsaicin has an unusually well-documented chemical lineage for a plant constituent.

First isolation and naming. The pungent principle of Capsicum was obtained in impure crystalline form and given the name capsaicin by J. C. Thresh in 1876, publishing in The Analyst; he followed it the next year with a fuller account of “capsaicin — the active principle of cayenne pepper” ^{45,46Reference 45Thresh JC · 1876CapsaicinView study →Reference 46Thresh JC · 1877Capsaicin — the active principle of cayenne pepperView study →}.

Structure. The constitution of the molecule was determined by E. K. Nelson in 1919, who established it as the vanillylamide of an unsaturated branched-chain fatty acid — the framework still written today as 8-methyl-N-vanillyl-trans-6-nonenamide ^{47Reference 47Nelson EK · 1919The constitution of capsaicin, the pungent principle of CapsicumView study →}.

First total synthesis. Ernst Späth and Stephen F. Darling published Synthese des Capsaicins in 1930, confirming Nelson’s structure by construction ^{48Reference 48Späth E · 1930Synthese des CapsaicinsView study →}.

The Scoville scale, published by Wilbur Scoville in 1912, sits between the isolation and the structure: a taste-dilution assay for pungency that predated any means of measuring the responsible molecule ^{49Reference 49Scoville WL · 1912Note on CapsicumsView study →}.

Manufacture today splits by purpose. Natural capsaicin is obtained by solvent extraction of Capsicum fruit as oleoresin capsicum, then purified — this is the route for pepper spray and for most food-grade material. Pharmaceutical-grade capsaicin for the 8% patch is a synthetic, high-purity material, because a licensed medicine cannot tolerate the cultivar-to-cultivar variability of an extract ^{5,12Reference 5Anand P · 2011Topical capsaicin for pain management: therapeutic potential and mechanisms of action of the new high-concentration capsaicin 8% patchView study →Reference 12Burness CB · 2016ReviewCapsaicin 8% patch: a review in peripheral neuropathic painView study →}.

Patents: not yet researched.

Commercial Production

Three distinct industries pull on this one molecule, and they want different things from it.

Food and colour. Oleoresin capsicum — a crude solvent extract of chilli fruit containing capsaicinoids alongside the carotenoid pigments — is a standard food ingredient, standardised by Scoville rating rather than by assayed capsaicin ^{49Reference 49Scoville WL · 1912Note on CapsicumsView study →}. Paprika oleoresin, extracted from non-pungent cultivars, supplies colour without heat.

Pharmaceutical. The 8% patch requires purified capsaicin at a controlled concentration and a validated impurity profile; the whole clinical case for the product rests on the dose being exactly what the label says ^{5,9Reference 5Anand P · 2011Topical capsaicin for pain management: therapeutic potential and mechanisms of action of the new high-concentration capsaicin 8% patchView study →Reference 9Derry S et al. · 2017Meta-analysisTopical capsaicin (high concentration) for chronic neuropathic pain in adultsView study →}.

Riot control and self-defence. Pepper spray is a pressurised Capsicum extract whose active component is capsaicin, deployed by police forces in many countries ^{40Reference 40Busker RW · 1998Toxicologic evaluation of pepper spray as a possible weapon for the Dutch police force: risk assessment and efficacyView study →}. This is by volume a significant use of the molecule and is discussed under Toxicity & Safety below, because it is the clearest demonstration of what concentrated capsaicin does to mucous membranes.

Toxicity & Safety

Capsaicin carries a moderate flag here. That is a deliberate step above the badge on the cayenne herb page, and it reflects the isolated molecule rather than the spice: this is a compound whose concentrated form is used as an incapacitating weapon, whose approved medical application is painful enough to require supervised administration, and for which there are documented case reports of acute harm from extreme-pungency ingestion. Systemic toxicity, by contrast, is genuinely low.

Local Irritation & Mucous-Membrane Exposure

The dominant hazard is intense local pain, and it is a direct consequence of the mechanism rather than a side effect of it. Contact with the eyes, nose, mouth, genitals or broken skin produces severe burning, involuntary eye closure, lacrimation and rhinorrhoea. Because capsaicin is lipophilic and water-insoluble, rinsing with water spreads rather than removes it. Application-site burning is the most common adverse effect of every topical preparation, and with low-concentration creams it is frequent enough to give a number needed to harm of 2.5 ^{13Reference 13Derry S · 2012Meta-analysisTopical capsaicin (low concentration) for chronic neuropathic pain in adultsView study →}.

Pepper Spray & Riot-Control Use

Pepper spray is Capsicum extract with capsaicin as the active component, and its effects are the same TRPV1 activation the medicine uses, delivered to the eyes and airway. A toxicological risk assessment for the Dutch police concluded pepper spray was relatively safe with a small risk of acute physical harm and that the risk of long-term health effects, including the much-discussed mutagenicity/carcinogenicity question, was negligible; deaths following pepper-spray use were largely attributed to other factors such as drugs and positional restraint — with the exception of one asthmatic man, in whose death the spray was judged contributory ^{40Reference 40Busker RW · 1998Toxicologic evaluation of pepper spray as a possible weapon for the Dutch police force: risk assessment and efficacyView study →}. Because capsaicin can induce bronchoconstriction, people with chronic obstructive lung disease may be hypersensitive, and the authors noted the experimental data were too few to draw firm conclusions ^{40Reference 40Busker RW · 1998Toxicologic evaluation of pepper spray as a possible weapon for the Dutch police force: risk assessment and efficacyView study →}. Forensic reviews of lethal riot-control-agent exposure make the same point about pre-existing airway disease ^{41Reference 41Toprak S et al. · 2015The pathology of lethal exposure to the Riot Control Agents: towards a forensics-based methodology for determining misuseView study →}.

Airway & Cough

Capsaicin is the standard agent for cough-provocation testing precisely because inhaling it reliably triggers the cough reflex in a dose-dependent way ^{44Reference 44Koskela HO et al. · 2021ReviewUtility of cough provocation tests in chronic cough and respiratory diseases: a comprehensive review and introduction of new reference rangesView study →}. That utility is the flip side of a hazard: aerosolised capsaicin — from cooking with dried chilli, from grinding, or from a spray — is a potent airway irritant, and the response is amplified in hyperreactive airways.

The Contested Gastric Question

This is the one place where the older safety literature has been substantially reversed. For decades chilli was believed to cause gastric ulcers, and ulcer patients were advised to avoid it. The evidence assembled since does not support that: capsaicin inhibits acid secretion and stimulates mucus, alkali and gastric mucosal blood flow, and epidemiology finds ulcers less common in higher chilli-consuming populations (adjusted OR 0.47) ^{27,28Reference 27Satyanarayana MN · 2006Capsaicin and gastric ulcersView study →Reference 28Kang JY et al. · 1995Chili — protective factor against peptic ulcer? Digestive Diseases and Sciences. https://pubmed.ncbi.nlm.nih.gov/7895548/View study →}. The honest summary is that capsaicin is not an established cause of peptic ulcer, and may be gastroprotective — but high oral doses still cause epigastric burning, heartburn and diarrhoea in many people, and that dose-dependent symptomatic irritation is real regardless of ulcer risk.

Ultra-Hot Pepper Challenges

Extreme-pungency eating challenges have produced genuine, published harms. A man developed thunderclap headache with reversible cerebral vasoconstriction after eating a Carolina Reaper in a contest ^{42Reference 42Boddhula SK et al. · 2018Case reportAn unusual cause of thunderclap headache after eating the hottest pepper in the world — “The Carolina Reaper”View study →}, and spontaneous oesophageal rupture (Boerhaave syndrome) requiring surgery has been reported after ghost-pepper ingestion — mediated by forceful vomiting rather than by direct chemical injury ^{43Reference 43Arens A et al. · 2016Case reportEsophageal rupture after ghost pepper ingestionView study →}. These are case reports, not a population risk from eating spicy food, but they are the reason ultra-hot challenges should not be treated as harmless entertainment.

Systemic Exposure & Interactions

From the 8% patch, systemic exposure is negligible: quantifiable plasma levels in a minority of patients, a maximum ever observed of 17.8 ng/mL, and a 1.64-hour half-life — which is why the manufacturers and reviewers describe a low risk of systemic effects or drug–drug interactions ^{5,17Reference 5Anand P · 2011Topical capsaicin for pain management: therapeutic potential and mechanisms of action of the new high-concentration capsaicin 8% patchView study →Reference 17Babbar S et al. · 2009Pharmacokinetic analysis of capsaicin after topical administration of a high-concentration capsaicin patch to patients with peripheral neuropathic painView study →}. For high-dose oral capsaicin the picture is less settled: metabolism involves a wide panel of cytochrome P450 enzymes (CYP1A1, 1A2, 2B6, 2C8, 2C9, 2C19, 2D6, 2E1, 3A4) and generates reactive electrophilic intermediates trapped by glutathione ^{19Reference 19Reilly CA et al. · 2003Metabolism of capsaicin by cytochrome P450 produces novel dehydrogenated metabolites and decreases cytotoxicity to lung and liver cellsView study →}. No clinically characterised interaction follows from this, but it means an interaction assessment for concentrated oral capsaicin has not been done, and silence should not be read as safety.

Carcinogenicity — Genuinely Unresolved

Capsaicin’s cancer literature points both ways. The same body of work that reports apoptosis induction and tumour suppression also documents carcinogenic and co-carcinogenic activity in other models; the standard review is titled The two faces of capsaicin and declines to resolve it ^{32Reference 32Bode AM · 2011The two faces of capsaicinView study →}. Regulatory-style toxicology of Capsicum extract concluded the mutagenicity/carcinogenicity risk from pepper-spray exposure was negligible ^{40Reference 40Busker RW · 1998Toxicologic evaluation of pepper spray as a possible weapon for the Dutch police force: risk assessment and efficacyView study →}. The correct posture is that this is unsettled at the isolate level and that capsaicin should be presented neither as an anticancer agent nor as a carcinogen.

Reproductive Note

No dedicated reproductive-toxicity dataset exists for isolated capsaicin at therapeutic topical exposures, and given the patch’s negligible systemic absorption a systemic reproductive effect is implausible on kinetic grounds ^{17Reference 17Babbar S et al. · 2009Pharmacokinetic analysis of capsaicin after topical administration of a high-concentration capsaicin patch to patients with peripheral neuropathic painView study →}. Pregnancy and lactation decisions belong to the herb or product page where intake is actually decided, not to this molecule page; no isolate-level verdict is issued here.

Safety at a Glance

⚠️ Moderate toxicity

• Systemic toxicity is low. The 8% patch produces quantifiable plasma capsaicin in a minority of patients, a maximum ever recorded of 17.8 ng/mL, and a 1.64-hour half-life — too little, too briefly, for systemic effects ^{17Reference 17Babbar S et al. · 2009Pharmacokinetic analysis of capsaicin after topical administration of a high-concentration capsaicin patch to patients with peripheral neuropathic painView study →}.
• Not an established cause of gastric ulcers. The older ulcer-causation claim has been reversed; capsaicin inhibits acid secretion and raises mucosal blood flow, and higher chilli intake tracks lower ulcer odds ^{27,28Reference 27Satyanarayana MN · 2006Capsaicin and gastric ulcersView study →Reference 28Kang JY et al. · 1995Chili — protective factor against peptic ulcer? Digestive Diseases and Sciences. https://pubmed.ncbi.nlm.nih.gov/7895548/View study →}.
• Severe mucous-membrane pain is the defining hazard. Eyes, airway, broken skin and mucosa react intensely; this is the same molecule used as pepper spray, and water does not wash it off ^{40Reference 40Busker RW · 1998Toxicologic evaluation of pepper spray as a possible weapon for the Dutch police force: risk assessment and efficacyView study →}.
• Airway irritation and bronchoconstriction. Inhaled capsaicin is the standard cough-provocation agent; people with asthma or COPD may be hypersensitive, and one pepper-spray death in an asthmatic was judged contributory ^{40,41,44Reference 40Busker RW · 1998Toxicologic evaluation of pepper spray as a possible weapon for the Dutch police force: risk assessment and efficacyView study →Reference 41Toprak S et al. · 2015The pathology of lethal exposure to the Riot Control Agents: towards a forensics-based methodology for determining misuseView study →Reference 44Koskela HO et al. · 2021ReviewUtility of cough provocation tests in chronic cough and respiratory diseases: a comprehensive review and introduction of new reference rangesView study →}.
• Topical burning is the mechanism, not a defect. Application-site burning is near-universal; low-concentration creams have an NNH of 2.5, and the 8% patch requires supervised application with anaesthesia or cooling ^{9,13Reference 9Derry S et al. · 2017Meta-analysisTopical capsaicin (high concentration) for chronic neuropathic pain in adultsView study →Reference 13Derry S · 2012Meta-analysisTopical capsaicin (low concentration) for chronic neuropathic pain in adultsView study →}.
• Ultra-hot pepper challenges have caused real harm. Published cases of thunderclap headache with cerebral vasoconstriction and of oesophageal rupture after extreme-pungency ingestion ^{42,43Reference 42Boddhula SK et al. · 2018Case reportAn unusual cause of thunderclap headache after eating the hottest pepper in the world — “The Carolina Reaper”View study →Reference 43Arens A et al. · 2016Case reportEsophageal rupture after ghost pepper ingestionView study →}.
• High-dose oral GI symptoms and uncharacterised interactions. Epigastric burning and diarrhoea are common at high oral doses, and concentrated oral capsaicin is metabolised by a broad panel of CYP enzymes with no clinical interaction assessment ^{19Reference 19Reilly CA et al. · 2003Metabolism of capsaicin by cytochrome P450 produces novel dehydrogenated metabolites and decreases cytotoxicity to lung and liver cellsView study →}.
• Carcinogenicity is unresolved. The preclinical literature contains both anti-tumour and tumour-promoting findings for the isolate ^{32Reference 32Bode AM · 2011The two faces of capsaicinView study →}.

Dosage

There is no established oral dose of isolated capsaicin, and no route on which it should be self-administered at pharmaceutical concentration. The figures below are what has been used in research and in licensed products.

The 8% patch is not a stronger version of the cream. It is roughly 100 times the concentration, applied once rather than repeatedly, and it is a supervised prescription procedure — the initial burn is severe enough that local anaesthetic or cooling is routinely used ^{5,9Reference 5Anand P · 2011Topical capsaicin for pain management: therapeutic potential and mechanisms of action of the new high-concentration capsaicin 8% patchView study →Reference 9Derry S et al. · 2017Meta-analysisTopical capsaicin (high concentration) for chronic neuropathic pain in adultsView study →}. It is not something that can be replicated with an OTC product or a home preparation.

Whole cayenne cannot be dosed to these figures. Capsaicin content of fruit varies from undetectable to ~0.4% of fresh weight between samples of the same genus ^{35,36Reference 35Al Othman ZA et al. · 2011Determination of capsaicin and dihydrocapsaicin in Capsicum fruit samples using high performance liquid chromatographyView study →Reference 36González-Zamora A et al. · 2013Characterization of different Capsicum varieties by evaluation of their capsaicinoids content by high performance liquid chromatography, determination of pungency and effect of high temperatureView study →}, which is exactly why every clinical figure above is stated in milligrams of purified isolate.

These are doses studied in research and used in licensed products, and are not a personal recommendation.

References

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