Thymoquinone

Thymoquinone at a Glance

Thymoquinone is a small monoterpenoid 1,4-benzoquinone and the signature marker compound of nigella — black seed, Nigella sativa. It is a minor component of the whole seed (roughly 0.01–0.4%) but the dominant constituent of the seed’s volatile oil, where it makes up something like a quarter to a third of the fraction ^{1,22Reference 1Ali BH · 2003ReviewPharmacological and toxicological properties of Nigella sativa — reviewView study →Reference 22Al-Ali A et al. · 2008AnimalOral and intraperitoneal LD50 of thymoquinone, an active principle of Nigella sativa, in mice and rats — animal studyView study →}. Essentially the entire modern pharmacological case for black seed is built on this one molecule ^{2,3Reference 2Gholamnezhad Z et al. · 2016ReviewPreclinical and clinical effects of Nigella sativa and its constituent, thymoquinone: a reviewView study →Reference 3Malik S et al. · 2021ReviewThymoquinone: a small molecule from nature with high therapeutic potential — reviewView study →}.

Its defining chemistry is that it is a quinone, not a phenol. Thymoquinone cycles between its oxidised quinone form and the reduced hydroquinone form, thymohydroquinone — which is why it behaves as an antioxidant in some settings and a deliberate pro-oxidant in others, and why the same molecule is credited with protecting healthy tissue and killing cancer cells ^{6,7,9Reference 6Badary OA et al. · 2003In vitroThymoquinone is a potent superoxide anion scavenger — in vitro studyView study →Reference 7Sakib R et al. · 2023In vitroAntioxidant properties of thymoquinone, thymohydroquinone and black cumin seed oil: scavenging of superoxide radical studied using cyclic voltammetry, DFT and single-crystal X-ray diffraction — in vitro studyView study →Reference 9Krylova NG et al. · 2019In vitroCytotoxic and antiproliferative effects of thymoquinone on rat C6 glioma cells depend on oxidative stress — in vitro studyView study →}. That biphasic behaviour is the honest mechanistic key to the compound, and it is more useful than the one-directional “powerful antioxidant” framing that dominates the popular literature.

Its defining practical problem is content. Thymoquinone is concentrated in the pressed oil rather than the plain seed powder, it degrades in light and in aqueous solution, and analytical surveys of commercial products have found ranges spanning more than two orders of magnitude between the weakest and strongest products on the same shelf ^{21,26,27Reference 21Salmani JM et al. · 2014In vitroAqueous solubility and degradation kinetics of the phytochemical anticancer thymoquinone; probing the effects of solvents, pH and light — in vitro studyView study →Reference 26Khaikin E et al. · 2022Screening of thymoquinone content in commercial Nigella sativa products to identify a promising and safe study medicationView study →Reference 27Alkhatib H et al. · 2020Thymoquinone content in marketed black seed oil in MalaysiaView study →}. Two black-seed trials are frequently not testing the same dose of anything.

The Redox Double Life

Almost every apparent contradiction in the thymoquinone literature resolves once you treat it as a redox-cycling quinone rather than a conventional radical scavenger.

In its oxidised (quinone) state, thymoquinone is an electron acceptor. Electrochemical and computational work shows it can directly oxidise superoxide back to molecular oxygen, forming a transient thymoquinone–superoxide radical species — and, notably, that the reduced partner thymohydroquinone does not scavenge superoxide under the same conditions ^{7Reference 7Sakib R et al. · 2023In vitroAntioxidant properties of thymoquinone, thymohydroquinone and black cumin seed oil: scavenging of superoxide radical studied using cyclic voltammetry, DFT and single-crystal X-ray diffraction — in vitro studyView study →}. This is the opposite of the usual phenol/hydroquinone antioxidant story and marks thymoquinone’s antioxidant behaviour as mechanistically unusual ^{6,7Reference 6Badary OA et al. · 2003In vitroThymoquinone is a potent superoxide anion scavenger — in vitro studyView study →Reference 7Sakib R et al. · 2023In vitroAntioxidant properties of thymoquinone, thymohydroquinone and black cumin seed oil: scavenging of superoxide radical studied using cyclic voltammetry, DFT and single-crystal X-ray diffraction — in vitro studyView study →}. In parallel, it induces the cell’s own antioxidant machinery through Nrf2, raising glutathione and phase-II enzymes such as DT-diaphorase (NQO1) — the very enzyme that two-electron-reduces quinones to hydroquinones, which is a self-referential loop worth noticing ^{5,8Reference 5Talebi M et al. · 2021ReviewBiological and therapeutic activities of thymoquinone: focus on the Nrf2 signaling pathway — reviewView study →Reference 8Mansour MA et al. · 2002AnimalEffects of thymoquinone on antioxidant enzyme activities, lipid peroxidation and DT-diaphorase in different tissues of mice: a possible mechanism of action — animal studyView study →}.

At higher concentrations the same chemistry runs the other way. Redox cycling between the quinone and hydroquinone forms generates reactive oxygen species, and cytotoxicity in glioma cells has been shown to depend on that oxidative stress rather than on any single receptor or pathway ^{9Reference 9Krylova NG et al. · 2019In vitroCytotoxic and antiproliferative effects of thymoquinone on rat C6 glioma cells depend on oxidative stress — in vitro studyView study →}. Its capacity as an electron-transfer mediator is direct enough that it has been used to convert type-II photosensitisers into type-I photosensitisers — a purely chemical demonstration of the same property ^{10Reference 10Zhuang J et al. · 2024Thymoquinone as an electron transfer mediator to convert type II photosensitizers to type I photosensitizersView study →}.

The cytoprotective arm is not merely theoretical: thymoquinone protects rat heart tissue against doxorubicin-induced cardiotoxicity, an injury driven largely by anthracycline redox cycling — one quinone buffering the damage of another ^{42Reference 42Nagi MN · 2000AnimalProtective effect of thymoquinone against doxorubicin-induced cardiotoxicity in rats: a possible mechanism of protection — animal studyView study →}. The practical read: the dose sets the direction. Low, dietary-scale exposure plausibly sits on the cytoprotective side; the concentrations that kill cancer cells in a dish sit on the pro-oxidant side and are not obviously reachable by swallowing seed oil. Any page that presents thymoquinone as simply “antioxidant” has thrown away the most interesting thing about it.

The Content Problem

This is the single most important caveat on the molecule, and it is an analytical one rather than a biological one.

Thymoquinone is a minor fraction of the seed (~0.01–0.4% by weight) and a major fraction of the volatile oil (~24–30%), so the preparation determines the dose far more than the quantity of plant material does ^{1,22Reference 1Ali BH · 2003ReviewPharmacological and toxicological properties of Nigella sativa — reviewView study →Reference 22Al-Ali A et al. · 2008AnimalOral and intraperitoneal LD50 of thymoquinone, an active principle of Nigella sativa, in mice and rats — animal studyView study →}. Cold-pressed and standardised oils deliver several-fold more thymoquinone per gram than raw ground seed, which is the most economical explanation for a recurring finding in the human trials: seed oil outperforms seed powder for fasting glucose and for total cholesterol and LDL-C in the meta-analytic subgroup analyses ^{28,29Reference 28Askari G et al. · 2019Meta-analysisEffect of Nigella sativa (black seed) supplementation on glycemic control: a systematic review and meta-analysis of clinical trialsView study →Reference 29Sahebkar A et al. · 2016Meta-analysisNigella sativa (black seed) effects on plasma lipid concentrations in humans: a systematic review and meta-analysis of randomized placebo-controlled trialsView study →}.

Commercial reality is worse than the botanical range suggests. An HPLC screen of six black seed oils and five black-seed capsules on the European market found thymoquinone contents from 3.08 to 809.4 mg/100 g — a spread of more than 260-fold, with only the best product plausibly delivering a research-relevant dose ^{26Reference 26Khaikin E et al. · 2022Screening of thymoquinone content in commercial Nigella sativa products to identify a promising and safe study medicationView study →}. An independent survey of the Malaysian market found 0.07% to 1.88% w/w, a 27-fold range, and concluded that several products simply cannot supply a practical amount ^{27Reference 27Alkhatib H et al. · 2020Thymoquinone content in marketed black seed oil in MalaysiaView study →}. Both papers argue the same thing: black-seed products should be regulated by thymoquinone content, because at present a consumer has no way to know what they are taking.

The consequence for evidence appraisal is blunt. A meta-analysis of “Nigella sativa” pools trials whose actual thymoquinone exposure may differ by two orders of magnitude, which is a large part of why heterogeneity in those pooled estimates is so consistently high ^{32Reference 32Jafari A et al. · 2025Meta-analysisDoes Nigella sativa supplementation improve cardiovascular disease risk factors? A comprehensive GRADE-assessed systematic review and dose-response meta-analysis of 82 randomized controlled trialsView study →}.

What It's Used For

The human evidence base is unusually large for a herbal constituent — more than eighty randomised trials have been pooled — but it is overwhelmingly evidence about black seed or black seed oil, not about the isolated molecule ^{32Reference 32Jafari A et al. · 2025Meta-analysisDoes Nigella sativa supplementation improve cardiovascular disease risk factors? A comprehensive GRADE-assessed systematic review and dose-response meta-analysis of 82 randomized controlled trialsView study →}. The isolate carries the mechanism story and two small trials; the plant carries the clinical record. The Rests on column below keeps that distinction explicit.

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. Scores here reflect the molecule’s case; where the human evidence is the whole seed or oil rather than the isolate, the score is held back accordingly.

01Glycaemic ControlPromising62%17-RCT and 82-RCT meta-analyses of seed/oil; one 60-patient isolate RCT as metformin add-on ^{28,32,33Reference 28Askari G et al. · 2019Meta-analysisEffect of Nigella sativa (black seed) supplementation on glycemic control: a systematic review and meta-analysis of clinical trialsView study →Reference 32Jafari A et al. · 2025Meta-analysisDoes Nigella sativa supplementation improve cardiovascular disease risk factors? A comprehensive GRADE-assessed systematic review and dose-response meta-analysis of 82 randomized controlled trialsView study →Reference 33Ali SM et al. · 2021RCTThymoquinone with metformin decreases fasting, post prandial glucose, and HbA1c in type 2 diabetic patients — randomized controlled trialView study →}

02Blood LipidsPromising58%17-RCT meta-analysis of seed/oil, oil > powder subgroup; 82-RCT GRADE synthesis ^{29,32Reference 29Sahebkar A et al. · 2016Meta-analysisNigella sativa (black seed) effects on plasma lipid concentrations in humans: a systematic review and meta-analysis of randomized placebo-controlled trialsView study →Reference 32Jafari A et al. · 2025Meta-analysisDoes Nigella sativa supplementation improve cardiovascular disease risk factors? A comprehensive GRADE-assessed systematic review and dose-response meta-analysis of 82 randomized controlled trialsView study →}

03Blood PressurePromising52%Two independent seed/oil meta-analyses; small effect, very high heterogeneity ^{30,31,32Reference 30Sahebkar A et al. · 2016Meta-analysisA systematic review and meta-analysis of randomized controlled trials investigating the effects of supplementation with Nigella sativa (black seed) on blood pressureView study →Reference 31Kavyani Z et al. · 2023Meta-analysisAntihypertensive effects of Nigella sativa supplementation: an updated systematic review and meta-analysis of randomized controlled trialsView study →Reference 32Jafari A et al. · 2025Meta-analysisDoes Nigella sativa supplementation improve cardiovascular disease risk factors? A comprehensive GRADE-assessed systematic review and dose-response meta-analysis of 82 randomized controlled trialsView study →}

04Inflammatory SignallingPromising48%Human biomarker pooling (CRP, IL-6, TNF-α) from seed/oil + potent isolate 5-LOX/NF-κB work ^{12,13,14,15,32Reference 12el-Dakhakhny M · 1965Studies on the Egyptian Nigella sativa LView study →Reference 13El-Dakhakhny M et al. · 2002In vitroNigella sativa oil, nigellone and derived thymoquinone inhibit synthesis of 5-lipoxygenase products in polymorphonuclear leukocytes from rats — in vitro studyView study →Reference 14Boudiaf K et al. · 2016AnimalThymoquinone strongly inhibits fMLF-induced neutrophil functions and exhibits anti-inflammatory properties in vivo — animal studyView study →Reference 15Wang D et al. · 2015In vitroThymoquinone inhibits IL-1β-induced inflammation in human osteoarthritis chondrocytes by suppressing NF-κB and MAPK signaling pathway — in vitro studyView study →Reference 32Jafari A et al. · 2025Meta-analysisDoes Nigella sativa supplementation improve cardiovascular disease risk factors? A comprehensive GRADE-assessed systematic review and dose-response meta-analysis of 82 randomized controlled trialsView study →}

05Asthma & AllergyShaky42%RCTs of seed/oil and of a 5%-thymoquinone standardised oil; meta-analysis of asthma control ^{35,36,37,38Reference 35Koshak A et al. · 2017RCTNigella sativa supplementation improves asthma control and biomarkers: a randomized, double-blind, placebo-controlled trialView study →Reference 36He T · 2020Meta-analysisThe influence of Nigella sativa for asthma control: a meta-analysisView study →Reference 37Kalus U et al. · 2003RCTEffect of Nigella sativa (black seed) on subjective feeling in patients with allergic diseases — randomized controlled trialView study →Reference 38Majeed A et al. · 2024RCTA randomized, double-blind, placebo-controlled study to evaluate the benefits of a standardized Nigella sativa oil containing 5% thymoquinone in reducing the symptoms of seasonal allergyView study →}

06AnticancerShaky30%Isolate cell-line and rodent work only; no human efficacy trial of any kind ^{16,17,18,19Reference 16He P et al. · 2023In vitroThymoquinone induces apoptosis and protective autophagy in gastric cancer cells by inhibiting the PI3K/Akt/mTOR pathway — in vitro studyView study →Reference 17Samarghandian S et al. · 2019In vitroThymoquinone-induced antitumor and apoptosis in human lung adenocarcinoma cells — in vitro studyView study →Reference 18Jehan S et al. · 2020In vitroThymoquinone selectively induces hepatocellular carcinoma cell apoptosis in synergism with clinical therapeutics and dependence of p53 status — in vitro studyView study →Reference 19Pang J et al. · 2017In vitroThymoquinone exerts potent growth-suppressive activity on leukemia through DNA hypermethylation reversal in leukemia cells — in vitro studyView study →}

07AnticonvulsantShaky26%One 22-child isolate crossover pilot + rodent seizure models ^{34Reference 34Akhondian J et al. · 2011RCTThe effect of thymoquinone on intractable pediatric seizures (pilot study) — randomized controlled trialView study →}

08AntimicrobialShaky24%In-vitro MIC work on the isolate, including from a non-Nigella source ^{43Reference 43Pradhan S et al. · 2026In vitroThymoquinone from lichen Dirinaria frostii (Tuck.): a new promising antimicrobial compound — in vitro studyView study →}

How Thymoquinone Works

Thymoquinone’s mechanism list is very long. What follows are the pathways with the most consistent independent support.

Nrf2 & the Phase-II Antioxidant Response

The best-supported cytoprotective route. Thymoquinone activates Nrf2, driving transcription of antioxidant and phase-II detoxification enzymes; in mice it raises glutathione, glutathione peroxidase, glutathione-S-transferase, catalase and quinone reductase (DT-diaphorase / NQO1) across several tissues while lowering lipid peroxidation ^{5,8Reference 5Talebi M et al. · 2021ReviewBiological and therapeutic activities of thymoquinone: focus on the Nrf2 signaling pathway — reviewView study →Reference 8Mansour MA et al. · 2002AnimalEffects of thymoquinone on antioxidant enzyme activities, lipid peroxidation and DT-diaphorase in different tissues of mice: a possible mechanism of action — animal studyView study →}. Notably, NQO1 is the enzyme that two-electron-reduces quinones — so thymoquinone induces the machinery that converts it into thymohydroquinone.

NF-κB & COX-2 Suppression

Thymoquinone suppresses NF-κB activation and downstream inflammatory transcription, reducing COX-2 and pro-inflammatory cytokine output; this is demonstrated in IL-1β-stimulated human chondrocytes alongside MAPK suppression ^{15Reference 15Wang D et al. · 2015In vitroThymoquinone inhibits IL-1β-induced inflammation in human osteoarthritis chondrocytes by suppressing NF-κB and MAPK signaling pathway — in vitro studyView study →}, and is the conventional explanation for the cytokine reductions seen in N. sativa trials ^{32Reference 32Jafari A et al. · 2025Meta-analysisDoes Nigella sativa supplementation improve cardiovascular disease risk factors? A comprehensive GRADE-assessed systematic review and dose-response meta-analysis of 82 randomized controlled trialsView study →}.

5-Lipoxygenase Inhibition

One of the most potent measured actions of the isolate: inhibition of 5-LOX product and 5-HETE formation in polymorphonuclear leukocytes at IC50 ≈ 0.26–0.36 µg/mL, roughly 100-fold more potent than the parent oil and about 40-fold more potent than nigellone, its polymer ^{13Reference 13El-Dakhakhny M et al. · 2002In vitroNigella sativa oil, nigellone and derived thymoquinone inhibit synthesis of 5-lipoxygenase products in polymorphonuclear leukocytes from rats — in vitro studyView study →}.

PI3K/Akt/mTOR & Apoptosis

In cancer cells, thymoquinone inhibits PI3K/Akt/mTOR signalling, inducing both apoptosis and protective autophagy ^{16Reference 16He P et al. · 2023In vitroThymoquinone induces apoptosis and protective autophagy in gastric cancer cells by inhibiting the PI3K/Akt/mTOR pathway — in vitro studyView study →}, with p53-dependent selectivity in hepatocellular models ^{18Reference 18Jehan S et al. · 2020In vitroThymoquinone selectively induces hepatocellular carcinoma cell apoptosis in synergism with clinical therapeutics and dependence of p53 status — in vitro studyView study →} and pro-apoptotic effects in lung adenocarcinoma lines ^{17Reference 17Samarghandian S et al. · 2019In vitroThymoquinone-induced antitumor and apoptosis in human lung adenocarcinoma cells — in vitro studyView study →}.

Epigenetic Effects

Thymoquinone reverses DNA hypermethylation in leukaemia cells, restoring expression of silenced genes — a mechanistically distinct route from its redox and kinase effects ^{19Reference 19Pang J et al. · 2017In vitroThymoquinone exerts potent growth-suppressive activity on leukemia through DNA hypermethylation reversal in leukemia cells — in vitro studyView study →}.

Direct Redox Chemistry

Underneath all of the above sits the quinone itself: superoxide oxidation via a transient thymoquinone–superoxide radical ^{6,7Reference 6Badary OA et al. · 2003In vitroThymoquinone is a potent superoxide anion scavenger — in vitro studyView study →Reference 7Sakib R et al. · 2023In vitroAntioxidant properties of thymoquinone, thymohydroquinone and black cumin seed oil: scavenging of superoxide radical studied using cyclic voltammetry, DFT and single-crystal X-ray diffraction — in vitro studyView study →}, reactive-oxygen generation through quinone/hydroquinone cycling ^{9Reference 9Krylova NG et al. · 2019In vitroCytotoxic and antiproliferative effects of thymoquinone on rat C6 glioma cells depend on oxidative stress — in vitro studyView study →}, and electron-transfer mediation demonstrable in pure chemistry ^{10Reference 10Zhuang J et al. · 2024Thymoquinone as an electron transfer mediator to convert type II photosensitizers to type I photosensitizersView study →}.

A Unifying Interpretation

The most economical reading is that thymoquinone is one chemical behaviour wearing many pharmacological hats. A small, lipophilic, membrane-permeant quinone that accepts and donates electrons will inevitably: (a) quench some radical species, (b) modify redox-sensitive cysteines on signalling proteins — which is exactly how NF-κB, Nrf2/Keap1 and several kinase cascades are regulated — and (c) generate reactive oxygen species once its local concentration exceeds the cell’s reducing capacity. Nrf2 activation, NF-κB suppression, 5-LOX inhibition, Akt inhibition and selective cancer-cell killing are all downstream-plausible consequences of that single property, at different doses and in different redox environments.

That framing explains the biphasic dose–response, explains why healthy and malignant cells respond differently, and explains why the mechanism list keeps growing. It also implies a limit: a molecule that works this way is unlikely to be a precise pharmacological tool, and its selectivity depends on a concentration window nobody has yet defined in humans.

The Promiscuity Problem

A caution that has to be stated plainly, because the thymoquinone literature invites it.

Thymoquinone has the structural profile of a compound that produces apparent hits across unrelated screens: it is redox-active, it is a Michael-acceptor-type electrophile reactive toward thiols, and it is membrane-active. In drug discovery these are precisely the properties that “nuisance substructure” and pan-assay-interference (PAINS) filters exist to flag, and an analysis of a comprehensive food-chemical database found such substructures — chemical reactivity, membrane disruption, redox cycling and colloidal aggregation — to be widespread among dietary compounds ^{11Reference 11Kaya I · 2020Analysis of nuisance substructures and aggregators in a comprehensive database of food chemical compoundsView study →}. The lesson from the curcumin literature is directly transferable: an implausibly broad mechanism list is itself weak evidence, not strong evidence.

Two further reasons for caution are specific to this field. First, the thymoquinone preclinical literature carries a documented retraction burden — multiple papers on thymoquinone’s neuroprotective, cardioprotective and anticancer effects have been retracted, so a single supporting citation should not be treated as settled. Second, much of the in-vitro work uses concentrations well above anything plausibly reached in plasma after oral dosing ^{20Reference 20Alkharfy KM et al. · 2015Pharmacokinetic plasma behaviors of intravenous and oral bioavailability of thymoquinone in a rabbit model — animal studyView study →}.

None of this means thymoquinone does nothing. It means the human data should carry most of the weight, and the human data are cardiometabolic, modest and mostly about the plant.

Pharmacokinetics

Thymoquinone’s pharmacokinetics have been characterised properly in only one animal model, and not at all in humans.

In rabbits given 5 mg/kg intravenously and 20 mg/kg orally, absolute oral bioavailability was ~58% with a lag time of ~23 minutes and a slow absorption half-life of ~217 minutes; elimination half-life was ~63 minutes after IV dosing and ~275 minutes after oral dosing, clearance ~7.2 mL/kg/min, and steady-state volume of distribution ~700 mL/kg. Plasma protein binding was estimated at over 99% ^{20Reference 20Alkharfy KM et al. · 2015Pharmacokinetic plasma behaviors of intravenous and oral bioavailability of thymoquinone in a rabbit model — animal studyView study →}. The profile is therefore slow in, fast out, with almost all circulating drug bound — a combination that makes free plasma concentrations far lower than total-drug figures suggest.

The formulation-relevant physical chemistry is often misstated. Thymoquinone’s aqueous solubility is not the main obstacle (>500 µg/mL, adequate for a parenteral dose); its stability is. It degrades rapidly in aqueous solution, particularly at alkaline pH, and is severely light-sensitive, with short light exposure causing heavy degradation independent of pH or solvent ^{21Reference 21Salmani JM et al. · 2014In vitroAqueous solubility and degradation kinetics of the phytochemical anticancer thymoquinone; probing the effects of solvents, pH and light — in vitro studyView study →}. That is the real reason nanoformulation is such an active area — lipid, polymeric and phytosome carriers are protecting the molecule and controlling its release rather than solubilising it ^{44Reference 44Modarresi Chahardehi A et al. · 2024ReviewNano-based formulations of thymoquinone are new approaches for psoriasis treatment: a literature reviewView study →}.

Human ADME — Cmax, half-life, oral bioavailability of the isolate — is not established. No published human pharmacokinetic study underpins the doses used in the two isolate trials.

Clinical Trials

Two controlled trials of isolated thymoquinone exist, plus a large body of trials of the plant.

The isolate trials are Ali et al. 2021 — thymoquinone 50 or 100 mg/day with metformin for 90 days in 60 people with type 2 diabetes ^{33Reference 33Ali SM et al. · 2021RCTThymoquinone with metformin decreases fasting, post prandial glucose, and HbA1c in type 2 diabetic patients — randomized controlled trialView study →} — and Akhondian et al. 2011, a double-blind crossover pilot of 1 mg/kg/day in 22 children with refractory epilepsy ^{34Reference 34Akhondian J et al. · 2011RCTThe effect of thymoquinone on intractable pediatric seizures (pilot study) — randomized controlled trialView study →}. Everything else is seed, seed powder or seed oil, including the COVID-19 trials of honey plus N. sativa ^{40Reference 40Ashraf S et al. · 2023RCTHoney and Nigella sativa against COVID-19 in Pakistan (HNS-COVID-PK): a multicenter placebo-controlled randomized clinical trialView study →} and their pooled mortality analysis ^{39Reference 39Kow CS et al. · 2024Meta-analysisThe effect of Nigella sativa on the risk of mortality in patients with COVID-19: a systematic review and meta-analysis of randomized trialsView study →}, which should be read against the general caution that pandemic-era herbal trials were frequently open-label and small.

Last checked: July 2026.

Isolate vs. Plant Studies

This is the crux of the thymoquinone file, and the split is unusually clean.

Evidence for the Isolate

The mechanistic literature is almost entirely the isolated molecule: the 5-LOX inhibition series ^{13Reference 13El-Dakhakhny M et al. · 2002In vitroNigella sativa oil, nigellone and derived thymoquinone inhibit synthesis of 5-lipoxygenase products in polymorphonuclear leukocytes from rats — in vitro studyView study →}, the neutrophil and NF-κB/MAPK work ^{14,15Reference 14Boudiaf K et al. · 2016AnimalThymoquinone strongly inhibits fMLF-induced neutrophil functions and exhibits anti-inflammatory properties in vivo — animal studyView study →Reference 15Wang D et al. · 2015In vitroThymoquinone inhibits IL-1β-induced inflammation in human osteoarthritis chondrocytes by suppressing NF-κB and MAPK signaling pathway — in vitro studyView study →}, the superoxide and DT-diaphorase studies ^{6,8Reference 6Badary OA et al. · 2003In vitroThymoquinone is a potent superoxide anion scavenger — in vitro studyView study →Reference 8Mansour MA et al. · 2002AnimalEffects of thymoquinone on antioxidant enzyme activities, lipid peroxidation and DT-diaphorase in different tissues of mice: a possible mechanism of action — animal studyView study →}, the redox-dependent cytotoxicity work ^{9Reference 9Krylova NG et al. · 2019In vitroCytotoxic and antiproliferative effects of thymoquinone on rat C6 glioma cells depend on oxidative stress — in vitro studyView study →} and the whole preclinical oncology corpus ^{16,17,18,19Reference 16He P et al. · 2023In vitroThymoquinone induces apoptosis and protective autophagy in gastric cancer cells by inhibiting the PI3K/Akt/mTOR pathway — in vitro studyView study →Reference 17Samarghandian S et al. · 2019In vitroThymoquinone-induced antitumor and apoptosis in human lung adenocarcinoma cells — in vitro studyView study →Reference 18Jehan S et al. · 2020In vitroThymoquinone selectively induces hepatocellular carcinoma cell apoptosis in synergism with clinical therapeutics and dependence of p53 status — in vitro studyView study →Reference 19Pang J et al. · 2017In vitroThymoquinone exerts potent growth-suppressive activity on leukemia through DNA hypermethylation reversal in leukemia cells — in vitro studyView study →}. Two small controlled human trials also used the isolate ^{33,34Reference 33Ali SM et al. · 2021RCTThymoquinone with metformin decreases fasting, post prandial glucose, and HbA1c in type 2 diabetic patients — randomized controlled trialView study →Reference 34Akhondian J et al. · 2011RCTThe effect of thymoquinone on intractable pediatric seizures (pilot study) — randomized controlled trialView study →}. As a mechanism base this is substantial; as a clinical base it is two studies.

Evidence for the Plant

The clinical record is the plant. All the cardiometabolic meta-analyses pool trials of N. sativa seed, seed powder or fixed oil ^{28,29,30,31,32Reference 28Askari G et al. · 2019Meta-analysisEffect of Nigella sativa (black seed) supplementation on glycemic control: a systematic review and meta-analysis of clinical trialsView study →Reference 29Sahebkar A et al. · 2016Meta-analysisNigella sativa (black seed) effects on plasma lipid concentrations in humans: a systematic review and meta-analysis of randomized placebo-controlled trialsView study →Reference 30Sahebkar A et al. · 2016Meta-analysisA systematic review and meta-analysis of randomized controlled trials investigating the effects of supplementation with Nigella sativa (black seed) on blood pressureView study →Reference 31Kavyani Z et al. · 2023Meta-analysisAntihypertensive effects of Nigella sativa supplementation: an updated systematic review and meta-analysis of randomized controlled trialsView study →Reference 32Jafari A et al. · 2025Meta-analysisDoes Nigella sativa supplementation improve cardiovascular disease risk factors? A comprehensive GRADE-assessed systematic review and dose-response meta-analysis of 82 randomized controlled trialsView study →}; the asthma and allergy trials are seed or oil preparations ^{35,36,37Reference 35Koshak A et al. · 2017RCTNigella sativa supplementation improves asthma control and biomarkers: a randomized, double-blind, placebo-controlled trialView study →Reference 36He T · 2020Meta-analysisThe influence of Nigella sativa for asthma control: a meta-analysisView study →Reference 37Kalus U et al. · 2003RCTEffect of Nigella sativa (black seed) on subjective feeling in patients with allergic diseases — randomized controlled trialView study →}; even the “5% thymoquinone” allergy trial is a standardised oil, not the molecule ^{38Reference 38Majeed A et al. · 2024RCTA randomized, double-blind, placebo-controlled study to evaluate the benefits of a standardized Nigella sativa oil containing 5% thymoquinone in reducing the symptoms of seasonal allergyView study →}. In almost none of these was the thymoquinone content of the intervention actually measured.

Why This Matters Here More Than Usual

For most constituents, isolate-versus-plant is a transferability caveat. Here it is a dosing problem: because thymoquinone content varies several-hundred-fold across products, “N. sativa 2 g/day” is not a thymoquinone dose at all ^{26,27Reference 26Khaikin E et al. · 2022Screening of thymoquinone content in commercial Nigella sativa products to identify a promising and safe study medicationView study →Reference 27Alkhatib H et al. · 2020Thymoquinone content in marketed black seed oil in MalaysiaView study →}. A trial reporting benefit and a trial reporting nothing may simply have used products differing tenfold in the active compound. Until trials report measured thymoquinone content, the plant literature cannot cleanly be read as evidence about this molecule.

Its Role in Plants

Thymoquinone belongs to the thymol/carvacrol branch of monoterpene chemistry — the same aromatic skeleton as p-cymene, oxidised further. In Lamiaceae, the route from γ-terpinene through p-cymene to thymol and carvacrol, and onward to thymohydroquinone, has been resolved: it proceeds via cytochrome P450s and a short-chain dehydrogenase/reductase, not the phenol-oxidase route long assumed ^{41Reference 41Krause ST et al. · 2021The biosynthesis of thymol, carvacrol, and thymohydroquinone in Lamiaceae proceeds via cytochrome P450s and a short-chain dehydrogenaseView study →}. Oxidation of thymohydroquinone then yields thymoquinone.

An important caveat: that pathway was characterised in Lamiaceae (thyme, oregano), while Nigella sativa is Ranunculaceae. The biosynthetic route in nigella itself has not been established at the enzyme level, and convergent evolution of this chemistry in an unrelated family is a genuinely interesting open question. Functionally, quinones of this type are typical defence chemistry — reactive, antimicrobial, deterrent to herbivores — and thymoquinone’s in-vitro antibacterial activity is consistent with that role ^{43Reference 43Pradhan S et al. · 2026In vitroThymoquinone from lichen Dirinaria frostii (Tuck.): a new promising antimicrobial compound — in vitro studyView study →}.

In the seed, thymoquinone does not sit alone. It exists in redox equilibrium with thymohydroquinone, and readily polymerises to nigellone (polythymoquinone) — a transformation el-Dakhakhny was already studying in the 1960s, comparing the “active principle” against its dihydro compound and its polymer ^{12Reference 12el-Dakhakhny M · 1965Studies on the Egyptian Nigella sativa LView study →}. The seed’s alkaloid fraction, nigellidine and nigellicine, is chemically unrelated.

Prevalence in Nature

Thymoquinone is not a widespread plant volatile. Its overwhelming natural association is with Nigella sativa seed, where it is the marker compound and the dominant volatile-oil constituent ^{1,22Reference 1Ali BH · 2003ReviewPharmacological and toxicological properties of Nigella sativa — reviewView study →Reference 22Al-Ali A et al. · 2008AnimalOral and intraperitoneal LD50 of thymoquinone, an active principle of Nigella sativa, in mice and rats — animal studyView study →}. It is reported in trace-to-minor amounts from some Lamiaceae with thymol/carvacrol chemistry — Monarda (bee balm) and Satureja (savory) species — but rarely as a major component, and this occurrence is much less well quantified than the Nigella figures.

The most surprising recent report is non-plant: thymoquinone was isolated and structurally confirmed (GC-MS, HPLC against standard, FTIR, ¹H and ¹³C NMR) from the lichen Dirinaria frostii, with antibacterial activity in MIC assays ^{43Reference 43Pradhan S et al. · 2026In vitroThymoquinone from lichen Dirinaria frostii (Tuck.): a new promising antimicrobial compound — in vitro studyView study →}. Lichens are fungus–alga symbioses, so this is a meaningfully different biosynthetic context and worth watching.

Within nigella, content depends on geographic origin, seed provenance, storage and extraction method, and is markedly higher in cold-pressed standardised oils than in raw powder ^{1Reference 1Ali BH · 2003ReviewPharmacological and toxicological properties of Nigella sativa — reviewView study →}. Storage matters because the molecule is light-sensitive and degrades ^{21Reference 21Salmani JM et al. · 2014In vitroAqueous solubility and degradation kinetics of the phytochemical anticancer thymoquinone; probing the effects of solvents, pH and light — in vitro studyView study →}.

Major Natural Sources

Source	Fraction	Reported thymoquinone content
Nigella (Nigella sativa)	whole seed	~0.01–0.4% w/w
Nigella (Nigella sativa)	volatile / ether-extract fraction	~24–30% of the fraction
Black seed oil — European market survey (11 products)	bottled oil & capsules	3.08–809.4 mg/100 g (~0.003–0.81% w/w)
Black seed oil — Malaysian market survey	bottled oil	0.07–1.88% w/w
Dirinaria frostii (lichen)	thallus extract	present; not quantified

The first two rows are botanical; rows three and four are what is actually on shop shelves, and the gap between them is the point ^{1,22,26,27,43Reference 1Ali BH · 2003ReviewPharmacological and toxicological properties of Nigella sativa — reviewView study →Reference 22Al-Ali A et al. · 2008AnimalOral and intraperitoneal LD50 of thymoquinone, an active principle of Nigella sativa, in mice and rats — animal studyView study →Reference 26Khaikin E et al. · 2022Screening of thymoquinone content in commercial Nigella sativa products to identify a promising and safe study medicationView study →Reference 27Alkhatib H et al. · 2020Thymoquinone content in marketed black seed oil in MalaysiaView study →Reference 43Pradhan S et al. · 2026In vitroThymoquinone from lichen Dirinaria frostii (Tuck.): a new promising antimicrobial compound — in vitro studyView study →}.

Discovery & Synthesis

Thymoquinone reached the pharmacological literature through the Egyptian pharmacologist Mahfouz el-Dakhakhny, whose Studies on the Egyptian Nigella sativa L. series in the 1960s identified the seeds’ “active principle” and compared its pharmacology directly against its dihydro compound (thymohydroquinone) and its polymer (nigellone) — a three-way comparison that already contained the redox and polymerisation story the modern literature rediscovered ^{12Reference 12el-Dakhakhny M · 1965Studies on the Egyptian Nigella sativa LView study →}. He was still publishing on the same three compounds four decades later, showing all of them inhibit 5-lipoxygenase ^{13Reference 13El-Dakhakhny M et al. · 2002In vitroNigella sativa oil, nigellone and derived thymoquinone inhibit synthesis of 5-lipoxygenase products in polymorphonuclear leukocytes from rats — in vitro studyView study →}. The name is straightforwardly descriptive: the quinone of thymol.

Chemically it is 2-isopropyl-5-methyl-1,4-benzoquinone, a small molecule readily prepared by oxidation of thymol, and synthetic thymoquinone is commercially available as an inexpensive research chemical — which is why the two isolate human trials were feasible at all. Most consumer exposure, however, remains via Nigella sativa seed and cold-pressed seed oil rather than the synthetic isolate ^{3,4Reference 3Malik S et al. · 2021ReviewThymoquinone: a small molecule from nature with high therapeutic potential — reviewView study →Reference 4Ahmad A et al. · 2019ReviewThymoquinone (2-isopropyl-5-methyl-1,4-benzoquinone) as a chemopreventive/anticancer agent: chemistry and biological effects — reviewView study →}.

Patents: not yet researched.

Stability & Formulation

Thymoquinone is an awkward molecule to make into a product, and this shapes both the research and the supplement market.

Light sensitivity is the dominant issue: short exposure causes severe degradation regardless of pH or solvent ^{21Reference 21Salmani JM et al. · 2014In vitroAqueous solubility and degradation kinetics of the phytochemical anticancer thymoquinone; probing the effects of solvents, pH and light — in vitro studyView study →}. Aqueous instability follows, with rapid first- or second-order degradation depending on pH and marked instability at alkaline pH — which rules out simple aqueous vehicles ^{21Reference 21Salmani JM et al. · 2014In vitroAqueous solubility and degradation kinetics of the phytochemical anticancer thymoquinone; probing the effects of solvents, pH and light — in vitro studyView study →}. Solubility, contrary to a widely repeated claim, is not the primary obstacle (>500 µg/mL) ^{21Reference 21Salmani JM et al. · 2014In vitroAqueous solubility and degradation kinetics of the phytochemical anticancer thymoquinone; probing the effects of solvents, pH and light — in vitro studyView study →}.

The formulation response has been nanoencapsulation — liposomes, lipospheres, polymeric and phytosome carriers — pursued across oncology, dermatology and neurology applications ^{44Reference 44Modarresi Chahardehi A et al. · 2024ReviewNano-based formulations of thymoquinone are new approaches for psoriasis treatment: a literature reviewView study →}. The stated goals are protection from degradation, controlled release and tissue targeting rather than solubilisation. None of these formulations has reached clinical use.

For consumers the practical implications are prosaic: buy oil in opaque or dark glass, store it cool and closed, and treat an old or clear-bottled product as likely depleted. This also compounds the content problem — the label figure, where one exists, was true on the day of manufacture.

Toxicity & Safety

Thymoquinone has a wide margin in animal toxicology and is well tolerated in the human trials that exist, but it is not inert, and the interaction data deserve more attention than they usually get.

Acute Toxicity

Oral LD50 in rodents is 870.9 mg/kg in mice and 794.3 mg/kg in rats; intraperitoneal LD50 is much lower, at 104.7 and 57.5 mg/kg respectively. The authors note these are 100–150 times (oral) and 10–15 times (intraperitoneal) the doses reported for anti-inflammatory, antioxidant and anticancer effects, and classed thymoquinone as relatively safe by the oral route ^{22Reference 22Al-Ali A et al. · 2008AnimalOral and intraperitoneal LD50 of thymoquinone, an active principle of Nigella sativa, in mice and rats — animal studyView study →}. Histopathology of liver, kidney, heart and lung was examined in the same study.

Cytochrome P450 Inhibition

This is the most concrete human-relevant risk. In human liver microsomes, thymoquinone inhibited all four major drug-metabolising CYPs tested, with an IC50 for CYP2C9 of ~0.5 µM — a strikingly low figure — alongside CYP1A2 (~26.5 µM), CYP3A4 (~25.2 µM) and CYP2D6 (>500 µM) ^{23Reference 23Albassam AA et al. · 2018In vitroInhibition of cytochrome P450 enzymes by thymoquinone in human liver microsomes — in vitro studyView study →}. The authors concluded a high probability of interaction with CYP-metabolised drugs, particularly CYP2C9 substrates. Preclinical follow-up has flagged specific risks with warfarin ^{24Reference 24Wang Z et al. · 2022Potential food-drug interaction risk of thymoquinone with warfarin — animal studyView study →} and phenytoin ^{25Reference 25Wang Z et al. · 2022Potential herb-drug interaction risk of thymoquinone and phenytoin — animal studyView study →}, both narrow-therapeutic-index CYP2C9 substrates. No human interaction study has been done, so the magnitude is unquantified — but the direction and the enzyme are identified.

Additive Pharmacological Effects

Because black seed reproducibly lowers glucose, blood pressure and lipids in pooled trials, thymoquinone-rich preparations can add to antidiabetic, antihypertensive and lipid-lowering drugs; the one isolate trial was itself an add-on to metformin ^{28,31,33Reference 28Askari G et al. · 2019Meta-analysisEffect of Nigella sativa (black seed) supplementation on glycemic control: a systematic review and meta-analysis of clinical trialsView study →Reference 31Kavyani Z et al. · 2023Meta-analysisAntihypertensive effects of Nigella sativa supplementation: an updated systematic review and meta-analysis of randomized controlled trialsView study →Reference 33Ali SM et al. · 2021RCTThymoquinone with metformin decreases fasting, post prandial glucose, and HbA1c in type 2 diabetic patients — randomized controlled trialView study →}. Monitoring for hypoglycaemia and hypotension is the sensible precaution.

The Pro-Oxidant Window

The same redox cycling that underwrites the anticancer work means thymoquinone is cytotoxic at sufficient concentration to normal cells too — selectivity in the published models is relative, not absolute, and depends on baseline redox state ^{9,18Reference 9Krylova NG et al. · 2019In vitroCytotoxic and antiproliferative effects of thymoquinone on rat C6 glioma cells depend on oxidative stress — in vitro studyView study →Reference 18Jehan S et al. · 2020In vitroThymoquinone selectively induces hepatocellular carcinoma cell apoptosis in synergism with clinical therapeutics and dependence of p53 status — in vitro studyView study →}. The concentration window has not been mapped in humans, which is a reason to be sceptical of high-dose isolate self-experimentation.

Provisional Ceiling

The most explicit dose ceiling in the literature comes from the German analytical group: based on the published literature they proposed a safe daily thymoquinone intake of under ~48.6 mg per adult, and noted that 4 mL/day of their best-tested oil supplies about 30 mg ^{26Reference 26Khaikin E et al. · 2022Screening of thymoquinone content in commercial Nigella sativa products to identify a promising and safe study medicationView study →}. This is a cautious, non-regulatory figure, not an established limit.

Reproductive and developmental data for isolated thymoquinone are limited; pregnancy and lactation are adjudicated per herb and per supplement, not on this molecule page.

Safety at a Glance

✅ Low / no toxicityDrug interactions

• Wide oral margin in animals. Rodent oral LD50 of ~795–870 mg/kg is 100–150× the doses used for its reported effects, and no acute toxicity signal emerged in the two small human isolate trials ^{22,33,34Reference 22Al-Ali A et al. · 2008AnimalOral and intraperitoneal LD50 of thymoquinone, an active principle of Nigella sativa, in mice and rats — animal studyView study →Reference 33Ali SM et al. · 2021RCTThymoquinone with metformin decreases fasting, post prandial glucose, and HbA1c in type 2 diabetic patients — randomized controlled trialView study →Reference 34Akhondian J et al. · 2011RCTThe effect of thymoquinone on intractable pediatric seizures (pilot study) — randomized controlled trialView study →}.
• Potent CYP2C9 inhibition in human liver microsomes (IC50 ≈ 0.5 µM), with flagged preclinical interaction risk for warfarin and phenytoin — narrow-therapeutic-index drugs where a shift matters ^{23,24,25Reference 23Albassam AA et al. · 2018In vitroInhibition of cytochrome P450 enzymes by thymoquinone in human liver microsomes — in vitro studyView study →Reference 24Wang Z et al. · 2022Potential food-drug interaction risk of thymoquinone with warfarin — animal studyView study →Reference 25Wang Z et al. · 2022Potential herb-drug interaction risk of thymoquinone and phenytoin — animal studyView study →}.
• You almost certainly don’t know your dose. Commercial black-seed products have been measured across a >260-fold range of thymoquinone content, and the molecule degrades in light ^{21,26,27Reference 21Salmani JM et al. · 2014In vitroAqueous solubility and degradation kinetics of the phytochemical anticancer thymoquinone; probing the effects of solvents, pH and light — in vitro studyView study →Reference 26Khaikin E et al. · 2022Screening of thymoquinone content in commercial Nigella sativa products to identify a promising and safe study medicationView study →Reference 27Alkhatib H et al. · 2020Thymoquinone content in marketed black seed oil in MalaysiaView study →}.
• Additive with antidiabetic, antihypertensive and lipid-lowering drugs — the pooled human effects are real if modest, so monitor glucose and blood pressure when combining ^{28,31,32Reference 28Askari G et al. · 2019Meta-analysisEffect of Nigella sativa (black seed) supplementation on glycemic control: a systematic review and meta-analysis of clinical trialsView study →Reference 31Kavyani Z et al. · 2023Meta-analysisAntihypertensive effects of Nigella sativa supplementation: an updated systematic review and meta-analysis of randomized controlled trialsView study →Reference 32Jafari A et al. · 2025Meta-analysisDoes Nigella sativa supplementation improve cardiovascular disease risk factors? A comprehensive GRADE-assessed systematic review and dose-response meta-analysis of 82 randomized controlled trialsView study →}.
• Pro-oxidant at higher concentrations. Selectivity for stressed or malignant cells is relative, not absolute, and the human concentration window is undefined ^{9,18Reference 9Krylova NG et al. · 2019In vitroCytotoxic and antiproliferative effects of thymoquinone on rat C6 glioma cells depend on oxidative stress — in vitro studyView study →Reference 18Jehan S et al. · 2020In vitroThymoquinone selectively induces hepatocellular carcinoma cell apoptosis in synergism with clinical therapeutics and dependence of p53 status — in vitro studyView study →}.
• The mechanism literature is broad and partly unreliable. Redox-cycling, thiol-reactive compounds generate assay artefacts, and this field carries a documented retraction burden ^{11Reference 11Kaya I · 2020Analysis of nuisance substructures and aggregators in a comprehensive database of food chemical compoundsView study →}.

Dosage

There is no established human dose for isolated thymoquinone, and no human pharmacokinetic study to anchor one. The figures below are the doses used in research, not recommendations.

Reading a Black Seed Oil Label

Because most human evidence is the oil, the practical question is how much thymoquinone an oil delivers. In the European market survey, 4 mL/day of the best-performing bottled oil supplied about 30 mg of thymoquinone, while the weakest products in the same survey supplied a small fraction of that ^{26Reference 26Khaikin E et al. · 2022Screening of thymoquinone content in commercial Nigella sativa products to identify a promising and safe study medicationView study →}. A product that does not state a measured thymoquinone percentage cannot be converted to a dose at all.

These are doses studied in research and are not a personal recommendation.

References

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