Allicin

Allicin at a Glance

Allicin (diallyl thiosulfinate) is the molecule responsible for the smell of freshly crushed garlic and for most of what garlic does in a petri dish. It is a small, volatile, highly reactive sulfur compound — a reactive sulfur species — that modifies accessible cysteine residues in proteins and depletes cellular thiol pools ^{1,21Reference 1Borlinghaus J et al. · 2014ReviewAllicin: chemistry and biological properties (review)View study →Reference 21Borlinghaus J et al. · 2021ReviewAllicin, the odor of freshly crushed garlic: a review of recent progress in understanding allicin’s effects on cellsView study →}.

Its defining fact is chemical rather than pharmacological: allicin is not present in intact garlic. A whole clove stores the odourless amino-acid derivative alliin in the cytoplasm and the enzyme alliinase in the vacuole. Damage the tissue and the two meet; allicin appears within a fraction of a second ^{5Reference 5Lawson LD · 1992Characterization of the formation of allicin and other thiosulfinates from garlicView study →}. It then begins to fall apart, rearranging into ajoene, the vinyldithiins, diallyl disulfide, diallyl trisulfide and diallyl sulfide ^{1,8Reference 1Borlinghaus J et al. · 2014ReviewAllicin: chemistry and biological properties (review)View study →Reference 8Lawson LD et al. · 1991Identification and HPLC quantitation of the sulfides and dialk(en)yl thiosulfinates in commercial garlic productsView study →}.

Everything practical about garlic follows from that instability. Allicin’s in-vitro antimicrobial breadth is genuine and well documented, but intact allicin is essentially undetectable in blood after swallowing garlic — the sulfur is absorbed, the molecule is not ^{40,41,12Reference 40Egen-Schwind C et al. · 1992AnimalMetabolism of garlic constituents in the isolated perfused rat liverView study →Reference 41Lachmann G et al. · 1994AnimalThe pharmacokinetics of the ³⁵S-labelled garlic constituents alliin, allicin and vinyldithiine (rats)View study →Reference 12Lawson LD · 2005Clinical trialAllicin and allicin-derived garlic compounds increase breath acetone through allyl methyl sulfide: use in measuring allicin bioavailability in human subjectsView study →}. Read the antimicrobial literature with that in mind: much of it uses concentrations that no oral dose is known to reach in tissue.

The Molecule That Does Not Exist Until You Crush It

An intact garlic clove contains no allicin at all. What it contains is the storage compound alliin (S-allyl-L-cysteine sulfoxide) — odourless, water-soluble, stable, at roughly 0.9% of fresh weight ^{7Reference 7Iberl B et al. · 1990Quantitative determination of allicin and alliin from garlic by HPLCView study →} — held apart from the pyridoxal-phosphate enzyme alliinase, one of the most abundant proteins in the bulb ^{6Reference 6Kuettner EB et al. · 2002Purification, characterization, and crystallization of alliinase from garlicView study →}.

Crushing, chopping or chewing ruptures the compartments. Alliinase cleaves alliin to allyl sulfenic acid plus ammonia and pyruvate, and two sulfenic acid molecules condense to allicin. Under laboratory conditions at 37 °C the reaction is complete in about 0.3 minutes — under twenty seconds ^{5Reference 5Lawson LD · 1992Characterization of the formation of allicin and other thiosulfinates from garlicView study →}. That is why garlic smells of nothing until it is cut.

The reaction then reverses its own direction of travel. Allicin is thermodynamically unstable and decomposes continuously:

• In water or dilute alcohol at room temperature its half-life is on the order of days — about 6 days by antibacterial bioassay and 11 days by chemical measurement ^{14Reference 14Fujisawa H et al. · 2008Biological and chemical stability of garlic-derived allicinView study →}.
• In vegetable oil it collapses in hours — an activity half-life of roughly 0.8 h and a chemical half-life of 3.1 h — which is why oil macerates contain ajoene and vinyldithiins rather than allicin, and steam-distilled garlic oil contains only the diallyl sulfides ^{14,8Reference 14Fujisawa H et al. · 2008Biological and chemical stability of garlic-derived allicinView study →Reference 8Lawson LD et al. · 1991Identification and HPLC quantitation of the sulfides and dialk(en)yl thiosulfinates in commercial garlic productsView study →}.
• Heat accelerates the decomposition; garlic that is heated before it is crushed loses the activity entirely because alliinase is denatured first ^{13,15Reference 13Cavagnaro PF et al. · 2007Effect of cooking on garlic (Allium sativum L.) antiplatelet activity and thiosulfinates contentView study →Reference 15Fujisawa H et al. · 2008Thermostability of allicin determined by chemical and biological assaysView study →}.

So there is no such thing as a garlic product with a long allicin shelf life. What products actually contain is either the precursor system (alliin plus active alliinase), or allicin’s decomposition products. Aged garlic extract is a third case entirely: aging converts the precursor pool towards the stable, water-soluble S-allylcysteine, and aged extracts carry little to no allicin ^{8,11Reference 8Lawson LD et al. · 1991Identification and HPLC quantitation of the sulfides and dialk(en)yl thiosulfinates in commercial garlic productsView study →Reference 11Lawson LD · 2005Clinical trialComposition, stability, and bioavailability of garlic products used in a clinical trialView study →}.

Allicin Potential vs. Allicin Delivered

This is the single most useful practical distinction on the page, and it is the reason garlic supplement performance varies wildly.

Almost no garlic supplement is standardised to allicin. It cannot be — the molecule would not survive the shelf. Products are standardised to allicin potential (also called allicin yield): the amount of allicin the tablet could generate if its alliin were fully converted by its alliinase ^{9Reference 9Lawson LD · 2001Low allicin release from garlic supplements: a major problem due to the sensitivities of alliinase activityView study →}. Two things then have to go right in the body for that potential to become real:

1. The alliinase has to survive the stomach. Below about pH 3.6 no thiosulfinates form at all, and — critically — the inhibition is irreversible: neutralising the pH afterwards does not restore activity ^{5Reference 5Lawson LD · 1992Characterization of the formation of allicin and other thiosulfinates from garlicView study →}. Gastric acid therefore destroys the enzyme outright, which is why serious garlic powder tablets are enteric coated to carry it past the stomach.
2. The tablet has to disintegrate fast enough. In a survey of all 24 enteric-coated brands then on the market, nearly every brand met its stated allicin potential when crushed in water — yet 83% released less than 15% of that potential under simulated gastrointestinal conditions, undone by excipients that impaired alliinase and by slow tablet disintegration ^{9Reference 9Lawson LD · 2001Low allicin release from garlic supplements: a major problem due to the sensitivities of alliinase activityView study →}. The authors’ conclusion was blunt: garlic powder supplements should be standardised on dissolution allicin release, not on allicin potential.

Later bioassay work using breath allyl methyl sulfide (see Pharmacokinetics) refined the picture and produced some genuinely counter-intuitive results. Enteric tablets delivered 36–104% of the bioavailability of raw crushed garlic — but only 22–57% when taken with a high-protein meal, because slower gastric emptying held them in acid for longer. Non-enteric tablets did better (80–111%), garlic powder capsules ranged 26–109%, and the Kwai garlic powder tablets used in a great many clinical trials came in at 80%, which retrospectively validates those trials as representing raw garlic ^{10Reference 10Lawson LD · 2018Clinical trialAllicin bioavailability and bioequivalence from garlic supplements and garlic foods (human volunteers)View study →}. Cooked and acidified garlic foods, which have no alliinase at all, still gave more than expected — boiled 16%, roasted 30%, pickled 19%, acid-minced 66%, black garlic 5% — partly because γ-glutamyl-S-allylcysteine and S-allylcysteine are metabolised down the same pathway ^{10Reference 10Lawson LD · 2018Clinical trialAllicin bioavailability and bioequivalence from garlic supplements and garlic foods (human volunteers)View study →}.

The practical reading: an “allicin potential” number on a label tells you very little on its own, an enteric coat helps but interacts badly with a heavy meal, and a well-made non-enteric garlic powder can outperform a badly-made enteric one.

Preparation & Cooking

Because the whole system is enzymatic, preparation is not a detail — it is the variable that decides whether there is any allicin at all.

The cleanest demonstration comes from an antiplatelet study that heated garlic crushed versus uncrushed. Oven-heating at 200 °C or boiling for three minutes or less did not reduce garlic’s ability to inhibit platelet aggregation. Heating for six minutes completely abolished the activity in uncrushed cloves — but crushed-then-heated samples retained reduced yet significant activity. Microwaved uncrushed garlic had no antiplatelet effect at all, and adding a little raw garlic juice back restored it. Throughout, activity tracked allicin and pyruvate levels ^{13Reference 13Cavagnaro PF et al. · 2007Effect of cooking on garlic (Allium sativum L.) antiplatelet activity and thiosulfinates contentView study →}.

The rule that falls out of this: crush first, then wait, then cook. Crushing generates the thiosulfinates before heat can denature alliinase; heating an intact clove destroys the enzyme before it ever meets its substrate. Prolonged cooking (beyond ~10 minutes) suppresses the activity either way, and the authors note the partial loss in crushed-cooked garlic can be offset simply by eating more ^{13Reference 13Cavagnaro PF et al. · 2007Effect of cooking on garlic (Allium sativum L.) antiplatelet activity and thiosulfinates contentView study →}.

What It's Used For

Allicin’s evidence base has an unusual shape: the isolate is exceptionally well studied in vitro, while every meaningful human result belongs to a garlic preparation whose allicin delivery is inferred rather than measured.

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. This molecule scores oddly by design: the isolate has enormous in-vitro volume at concentrations that are not systemically achievable, while the human data belong to whole garlic preparations. Read the per-row basis.

01Antimicrobial Activity In Vitro and TopicalShaky42%Extensive isolate in-vitro work + defined thiol mechanism; no systemic human trials ^{17,19,20,22,24Reference 17Rabinkov A et al. · 1998In vitroThe mode of action of allicin: trapping of radicals and interaction with thiol containing proteins (in vitro)View study →Reference 19Müller A et al. · 2016In vitroAllicin induces thiol stress in bacteria through S-allylmercapto modification of protein cysteines (in vitro)View study →Reference 20Reiter J et al. · 2020In vitroAllicin, a natural antimicrobial defence substance from garlic, inhibits DNA gyrase activity in bacteria (in vitro)View study →Reference 22Choo S et al. · 2020ReviewReview: antimicrobial properties of allicin used alone or in combination with other medicationsView study →Reference 24Cutler RR · 2004In vitroAntibacterial activity of a new, stable, aqueous extract of allicin against methicillin-resistant Staphylococcus aureus (in vitro)View study →}

02Blood Pressure Garlic PreparationsShaky40%Human meta-analyses of garlic preparations, not isolate ^{30,31Reference 30Ried K · 2016Meta-analysisGarlic lowers blood pressure in hypertensive individuals, regulates serum cholesterol, and stimulates immunity: an updated meta-analysis and reviewView study →Reference 31Stabler SN et al. · 2012Systematic reviewGarlic for the prevention of cardiovascular morbidity and mortality in hypertensive patients (Cochrane systematic review)View study →}

03Antiplatelet and AntithromboticShaky34%Ex-vivo human platelet assays identifying allicin as the driver ^{37,13Reference 37Lawson LD et al. · 1992Inhibition of whole blood platelet-aggregation by compounds in garlic clove extracts and commercial garlic productsView study →Reference 13Cavagnaro PF et al. · 2007Effect of cooking on garlic (Allium sativum L.) antiplatelet activity and thiosulfinates contentView study →}

04Serum Lipids Garlic PreparationsShaky26%Positive meta-analyses undercut by a well-controlled null RCT ^{32,33Reference 32Ried K et al. · 2013Meta-analysisEffect of garlic on serum lipids: an updated meta-analysisView study →Reference 33Gardner CD et al. · 2007RCTEffect of raw garlic vs commercial garlic supplements on plasma lipid concentrations in adults with moderate hypercholesterolemia: a randomized clinical trialView study →}

05Common Cold PreventionShaky22%One allicin-standardised RCT; Cochrane calls the evidence insufficient ^{34,35Reference 34Josling P · 2001RCTPreventing the common cold with a garlic supplement: a double-blind, placebo-controlled surveyView study →Reference 35Lissiman E et al. · 2014Systematic reviewGarlic for the common cold (Cochrane systematic review)View study →}

06Gastric Cancer ChemopreventionShaky18%Observational meta-analysis of garlic intake; mechanism belongs to the sulfides ^{36Reference 36Kodali RT · 2015Meta-analysisMeta-analysis: does garlic intake reduce risk of gastric cancer? Nutrition and Cancer. https://pubmed.ncbi.nlm.nih.gov/25411831/View study →}

07Anticancer Cell and Animal ModelsShaky14%Cell lines only; effects are dose-dependent cytotoxicity ^{1,21Reference 1Borlinghaus J et al. · 2014ReviewAllicin: chemistry and biological properties (review)View study →Reference 21Borlinghaus J et al. · 2021ReviewAllicin, the odor of freshly crushed garlic: a review of recent progress in understanding allicin’s effects on cellsView study →}

Mechanisms

The load-bearing row is the first one. Allicin does not have a receptor; it has a chemistry. Everything below it is a consequence of the same reaction with whatever thiol it meets first.

Pharmacokinetics

This is the section that reframes the rest of the page, and it is unusually clear-cut.

Intact allicin does not reach the systemic circulation in any measurable way. In the isolated perfused rat liver, allicin showed a remarkable first-pass effect, passing through unmetabolised only at concentrations high enough to cause considerable hepatocyte injury; diallyl disulfide and allyl mercaptan were identified as its metabolites in perfusate, bile and liver tissue ^{40Reference 40Egen-Schwind C et al. · 1992AnimalMetabolism of garlic constituents in the isolated perfused rat liverView study →}. A ³⁵S radiolabel study in rats confirmed the pattern from the other end: after an 8 mg/kg oral dose, sulfur label was well absorbed (a minimum absorption rate of ~65%, with 85.5% of the dose recovered in urine and faeces by 72 h) — but unchanged allicin was absent from the urinary metabolite pattern entirely, indicating extensive metabolisation to highly polar acidic metabolites ^{41Reference 41Lachmann G et al. · 1994AnimalThe pharmacokinetics of the ³⁵S-labelled garlic constituents alliin, allicin and vinyldithiine (rats)View study →}. The same study found allicin and the vinyldithiins pharmacokinetically indistinguishable ^{41,42Reference 41Lachmann G et al. · 1994AnimalThe pharmacokinetics of the ³⁵S-labelled garlic constituents alliin, allicin and vinyldithiine (rats)View study →Reference 42Egen-Schwind C et al. · 1992Pharmacokinetics of vinyldithiins, transformation products of allicinView study →}.

Because the parent molecule cannot be tracked, the field measures a surrogate. Breath allyl methyl sulfide (AMS) rises after allicin ingestion, and the allyl thiosulfinates are solely responsible for it; diallyl trisulfide, diallyl disulfide, ajoene and S-allylmercaptocysteine all produce the same quantitative effect at equimolar dithioallyl, and AMS given directly reproduces it — so AMS is the main and an active metabolite of allicin, and its 32–48 h area-under-curve is the accepted proxy for “allicin bioavailability” ^{12,10Reference 12Lawson LD · 2005Clinical trialAllicin and allicin-derived garlic compounds increase breath acetone through allyl methyl sulfide: use in measuring allicin bioavailability in human subjectsView study →Reference 10Lawson LD · 2018Clinical trialAllicin bioavailability and bioequivalence from garlic supplements and garlic foods (human volunteers)View study →}. That is the measurement behind every bioavailability percentage in the Allicin Potential vs. Allicin Delivered section — and it is worth being precise about what it means: it shows the allyl sulfur got in, not that allicin did.

Two further constraints matter. First, allicin is quenched by thiols: adding cysteine or glutathione cancels its antibacterial activity ^{16Reference 16Fujisawa H et al. · 2009In vitroAntibacterial potential of garlic-derived allicin and its cancellation by sulfhydryl compounds (in vitro)View study →}, and human blood is full of both. Second, allicin is membrane-permeable and enters cells readily ^{18,21Reference 18Miron T et al. · 2000In vitroThe mode of action of allicin: its ready permeability through phospholipid membranes may contribute to its biological activity (in vitro)View study →Reference 21Borlinghaus J et al. · 2021ReviewAllicin, the odor of freshly crushed garlic: a review of recent progress in understanding allicin’s effects on cellsView study →} — which is why it works so well in a dish and why it has no selectivity to offer once inside a mammal.

Clinical Trials

There are no controlled human trials of isolated allicin. Every human result belongs to a garlic preparation: meta-analyses of garlic for blood pressure ^{30,52Reference 30Ried K · 2016Meta-analysisGarlic lowers blood pressure in hypertensive individuals, regulates serum cholesterol, and stimulates immunity: an updated meta-analysis and reviewView study →Reference 52Saadh MJ et al. · 2024Meta-analysisEffects of aged garlic extract on blood pressure in hypertensive patients: a systematic review and meta-analysis of randomized controlled trialsView study →} and serum lipids ^{32Reference 32Ried K et al. · 2013Meta-analysisEffect of garlic on serum lipids: an updated meta-analysisView study →}, a Cochrane review finding no adequate cardiovascular-endpoint trials ^{31Reference 31Stabler SN et al. · 2012Systematic reviewGarlic for the prevention of cardiovascular morbidity and mortality in hypertensive patients (Cochrane systematic review)View study →}, the well-characterised null lipid RCT comparing raw garlic with two supplement types ^{33Reference 33Gardner CD et al. · 2007RCTEffect of raw garlic vs commercial garlic supplements on plasma lipid concentrations in adults with moderate hypercholesterolemia: a randomized clinical trialView study →}, and the single allicin-standardised common-cold RCT that Cochrane found insufficient to conclude from ^{34,35Reference 34Josling P · 2001RCTPreventing the common cold with a garlic supplement: a double-blind, placebo-controlled surveyView study →Reference 35Lissiman E et al. · 2014Systematic reviewGarlic for the common cold (Cochrane systematic review)View study →}. Two supporting studies characterise what the trial materials actually delivered ^{11,10Reference 11Lawson LD · 2005Clinical trialComposition, stability, and bioavailability of garlic products used in a clinical trialView study →Reference 10Lawson LD · 2018Clinical trialAllicin bioavailability and bioequivalence from garlic supplements and garlic foods (human volunteers)View study →} — a standard the field should apply more often.

Last checked: July 2026.

How Allicin Works

Allicin has one mechanism wearing many costumes. The S–S(=O) thiosulfinate group is an electrophilic sulfur centre: any accessible thiol attacks it, forming a mixed S-allylmercapto disulfide and releasing allyl sulfenic acid. That single reaction accounts for essentially everything below.

Thiol–disulfide exchange with protein cysteines. Allicin S-thioallylates accessible cysteine residues in proteins, and low-molecular-weight thiols including glutathione in eukaryotes and Gram-negative bacteria and bacillithiol in Gram-positive firmicutes ^{21,17Reference 21Borlinghaus J et al. · 2021ReviewAllicin, the odor of freshly crushed garlic: a review of recent progress in understanding allicin’s effects on cellsView study →Reference 17Rabinkov A et al. · 1998In vitroThe mode of action of allicin: trapping of radicals and interaction with thiol containing proteins (in vitro)View study →}. In E. coli, growth inhibition coincides with depletion of the glutathione pool, S-allylmercapto modification of cytoplasmic proteins and a fall in total sulfhydryl levels, accompanied by induction of the oxidative and heat-stress responses; the modified proteins aggregate, which stabilises RpoH and drives the heat-stress response ^{19Reference 19Müller A et al. · 2016In vitroAllicin induces thiol stress in bacteria through S-allylmercapto modification of protein cysteines (in vitro)View study →}.

Inactivation of specific enzymes. The same chemistry hits particular targets hard. Isocitrate lyase (AceA) is largely inhibited in vivo by allicin treatment ^{19Reference 19Müller A et al. · 2016In vitroAllicin induces thiol stress in bacteria through S-allylmercapto modification of protein cysteines (in vitro)View study →}. More strikingly, DNA gyrase is an allicin target: Cys433 of the GyrA subunit goes from ~6% oxidised to 55% in allicin-susceptible Pseudomonas, and allicin inhibits E. coli gyrase in vitro in the same concentration range as nalidixic acid ^{20Reference 20Reiter J et al. · 2020In vitroAllicin, a natural antimicrobial defence substance from garlic, inhibits DNA gyrase activity in bacteria (in vitro)View study →}.

Membrane permeability. Allicin passes readily through phospholipid membranes, which is what lets a small, non-transported molecule reach intracellular targets at all ^{18Reference 18Miron T et al. · 2000In vitroThe mode of action of allicin: its ready permeability through phospholipid membranes may contribute to its biological activity (in vitro)View study →}. In whole bacteria, allicin-containing garlic extracts also cause visible membrane disintegration and loss of structural integrity ^{26Reference 26Booyens J et al. · 2014In vitroIn vitro antibacterial mechanism of action of crude garlic (Allium sativum) clove extract on selected probiotic Bifidobacterium species as revealed by SEM, TEM and SDS-PAGE analysisView study →}.

Radical trapping. Independently of its thiol chemistry, allicin traps free radicals — the original mechanistic work paired radical trapping with thiol-protein interaction as its two arms ^{17Reference 17Rabinkov A et al. · 1998In vitroThe mode of action of allicin: trapping of radicals and interaction with thiol containing proteins (in vitro)View study →}.

Virulence-gene suppression. Beyond killing, sub-lethal allicin regulates genes essential for microbial virulence, and it potentiates conventional antibiotics and antifungals in combination ^{22Reference 22Choo S et al. · 2020ReviewReview: antimicrobial properties of allicin used alone or in combination with other medicationsView study →}.

A Unifying Interpretation

The thing to hold onto is that allicin’s breadth and its lack of selectivity are the same property.

A molecule that inactivates any accessible cysteine will inhibit bacteria, fungi, protozoa and Candida — and will also modify host proteins, deplete host glutathione, and induce apoptosis or necrosis in mammalian cells in a dose-dependent way ^{1,21Reference 1Borlinghaus J et al. · 2014ReviewAllicin: chemistry and biological properties (review)View study →Reference 21Borlinghaus J et al. · 2021ReviewAllicin, the odor of freshly crushed garlic: a review of recent progress in understanding allicin’s effects on cellsView study →}. There is no target that makes it a microbial-specific agent; the apparent selectivity in a petri dish is a selectivity of exposure, not of chemistry.

That same reactivity is why it is not a systemic drug. The reaction that makes allicin useful against a bacterium is the reaction that consumes it before it gets anywhere — quenched by cysteine and glutathione ^{16Reference 16Fujisawa H et al. · 2009In vitroAntibacterial potential of garlic-derived allicin and its cancellation by sulfhydryl compounds (in vitro)View study →}, extensively metabolised on first pass through the liver ^{40Reference 40Egen-Schwind C et al. · 1992AnimalMetabolism of garlic constituents in the isolated perfused rat liverView study →}, and absent from urine as the parent compound ^{41Reference 41Lachmann G et al. · 1994AnimalThe pharmacokinetics of the ³⁵S-labelled garlic constituents alliin, allicin and vinyldithiine (rats)View study →}.

So the honest synthesis is: allicin is a contact agent. It performs where it can meet its target directly and at concentration — on skin, on a surface, in the mouth, arguably in the gut lumen, and in the plant tissue it evolved to defend. Claims that route it through the bloodstream to a distant organ are asking it to do the one thing its chemistry rules out.

The In-Vitro Concentration Problem

Because so much of the popular case for garlic rests on antimicrobial papers, it is worth doing the arithmetic explicitly.

A typical clove weighs around 4 g and yields on the order of 0.4% allicin by fresh weight when crushed ^{7Reference 7Iberl B et al. · 1990Quantitative determination of allicin and alliin from garlic by HPLCView study →} — roughly 16 mg. Suppose, generously and counter-factually, that every milligram entered the bloodstream intact and distributed instantly through ~5 L of blood. That is about 3 µg/mL. The MIC for allicin against clinical MRSA isolates is 16 µg/mL, and the bactericidal concentrations are 128–256 µg/mL ^{24Reference 24Cutler RR · 2004In vitroAntibacterial activity of a new, stable, aqueous extract of allicin against methicillin-resistant Staphylococcus aureus (in vitro)View study →}.

So even the physically impossible best case falls several-fold short of the inhibitory concentration and roughly two orders of magnitude short of the bactericidal one. And the best case does not happen: allicin undergoes extensive first-pass metabolism ^{40Reference 40Egen-Schwind C et al. · 1992AnimalMetabolism of garlic constituents in the isolated perfused rat liverView study →}, is not recovered intact in urine ^{41Reference 41Lachmann G et al. · 1994AnimalThe pharmacokinetics of the ³⁵S-labelled garlic constituents alliin, allicin and vinyldithiine (rats)View study →}, and is chemically cancelled by the cysteine and glutathione present throughout blood and tissue ^{16Reference 16Fujisawa H et al. · 2009In vitroAntibacterial potential of garlic-derived allicin and its cancellation by sulfhydryl compounds (in vitro)View study →}.

None of this makes the antimicrobial literature wrong. It makes it route-specific. Allicin at 500 µg/mL in a topical cream is a coherent proposition ^{24Reference 24Cutler RR · 2004In vitroAntibacterial activity of a new, stable, aqueous extract of allicin against methicillin-resistant Staphylococcus aureus (in vitro)View study →}; allicin at 16 µg/mL in the bloodstream after dinner is not. Any page — including this one — that reports MIC values without that caveat is inviting a false inference.

Isolate vs. Plant Studies

The split here is unusually stark, and it runs the opposite way to most plant compounds.

The isolate owns the in-vitro evidence. Pure and stabilised allicin has been characterised directly: MIC/MBC panels against MRSA ^{24Reference 24Cutler RR · 2004In vitroAntibacterial activity of a new, stable, aqueous extract of allicin against methicillin-resistant Staphylococcus aureus (in vitro)View study →}, proteome-wide thiol-modification mapping ^{19Reference 19Müller A et al. · 2016In vitroAllicin induces thiol stress in bacteria through S-allylmercapto modification of protein cysteines (in vitro)View study →}, gyrase inhibition ^{20Reference 20Reiter J et al. · 2020In vitroAllicin, a natural antimicrobial defence substance from garlic, inhibits DNA gyrase activity in bacteria (in vitro)View study →}, membrane permeability ^{18Reference 18Miron T et al. · 2000In vitroThe mode of action of allicin: its ready permeability through phospholipid membranes may contribute to its biological activity (in vitro)View study →}, radical trapping and thiol-protein interaction ^{17Reference 17Rabinkov A et al. · 1998In vitroThe mode of action of allicin: trapping of radicals and interaction with thiol containing proteins (in vitro)View study →}, and synthesised allicin evaluated against its own transformation products ^{51Reference 51Ilić DP et al. · 2015In vitroBiological evaluation of synthesized allicin and its transformation products obtained by microwaves in methanol: antioxidant activity and effect on cell growth (in vitro)View study →}. This is genuinely isolate-level mechanism work of good quality.

The plant owns the human evidence — entirely. Every clinical result is for a garlic preparation: raw crushed clove, garlic powder tablets, aged garlic extract, an allicin-standardised capsule ^{33,30,32,34Reference 33Gardner CD et al. · 2007RCTEffect of raw garlic vs commercial garlic supplements on plasma lipid concentrations in adults with moderate hypercholesterolemia: a randomized clinical trialView study →Reference 30Ried K · 2016Meta-analysisGarlic lowers blood pressure in hypertensive individuals, regulates serum cholesterol, and stimulates immunity: an updated meta-analysis and reviewView study →Reference 32Ried K et al. · 2013Meta-analysisEffect of garlic on serum lipids: an updated meta-analysisView study →Reference 34Josling P · 2001RCTPreventing the common cold with a garlic supplement: a double-blind, placebo-controlled surveyView study →}. Two of these preparations (aged garlic extract, black garlic) contain essentially no allicin yet appear in the positive meta-analyses ^{52,10Reference 52Saadh MJ et al. · 2024Meta-analysisEffects of aged garlic extract on blood pressure in hypertensive patients: a systematic review and meta-analysis of randomized controlled trialsView study →Reference 10Lawson LD · 2018Clinical trialAllicin bioavailability and bioequivalence from garlic supplements and garlic foods (human volunteers)View study →} — which is a strong argument that garlic’s cardiovascular effect is not a pure allicin effect.

The transferability caveat therefore points in an unusual direction. It is not that whole-plant results are being over-attributed to a molecule that was never tested alone; allicin has been tested alone, extensively. It is that the isolate’s evidence sits at concentrations and by routes the plant cannot deliver, while the plant’s evidence sits at endpoints the isolate has never been tested against.

Its Role in Plants

Allicin is a damage-activated chemical defence, and the whole two-compartment design makes sense only in that light ^{1,21Reference 1Borlinghaus J et al. · 2014ReviewAllicin: chemistry and biological properties (review)View study →Reference 21Borlinghaus J et al. · 2021ReviewAllicin, the odor of freshly crushed garlic: a review of recent progress in understanding allicin’s effects on cellsView study →}.

Storing an inert precursor apart from its activating enzyme is a costly arrangement that buys the plant one thing: the toxin is produced only at the site and moment of injury. A pathogen’s hyphae or an insect’s mandibles rupture cells; allicin appears in under a second ^{5Reference 5Lawson LD · 1992Characterization of the formation of allicin and other thiosulfinates from garlicView study →}, at high local concentration, exactly where it is needed — and then decays, so the plant is not permanently poisoning itself. Allicin also inhibits seed germination and attenuates root development in other plants, so the same chemistry has an allelopathic dimension ^{1Reference 1Borlinghaus J et al. · 2014ReviewAllicin: chemistry and biological properties (review)View study →}.

Garlic runs two alliinase activities with different substrate preferences and different pH optima: the diallyl thiosulfinates (allicin and its 1-propenyl relatives) form optimally at pH 4.5–5.0 and complete in ~0.3 min, while the methyl thiosulfinates form optimally at pH 6.5–7.0 and take ~3.5 min ^{5Reference 5Lawson LD · 1992Characterization of the formation of allicin and other thiosulfinates from garlicView study →}. Alliinase itself is a pyridoxal-5′-phosphate-dependent C–S lyase, abundant enough in the bulb to be purified and crystallised readily ^{6Reference 6Kuettner EB et al. · 2002Purification, characterization, and crystallization of alliinase from garlicView study →}.

Prevalence in Nature

Allicin is, for practical purposes, a garlic molecule — but the underlying capability is a genus-level trait. What varies across Allium is which cysteine sulfoxide dominates, and allicin is only produced where alliin does.

A survey of cysteine sulfoxides across the genus found methiin present in every species examined, with the other sulfoxides varying in a pattern that tracks culinary use. Two major chemotypes emerge: the “onion type”, dominated by isoalliin (onion, chive, top onion), and the “garlic type”, dominated by alliin — which besides A. sativum includes wild leek (A. obliquum) and sand leek (A. scorodoprasum). Pearl onion and leek (A. ampeloprasum) run higher on methiin and propiin. Ramsons (A. ursinum) is unusual in carrying a roughly equal triple mix of methiin, alliin and isoalliin ^{43Reference 43Fritsch RM · 2006Occurrence and taxonomic significance of cysteine sulphoxides in the genus Allium L. (Alliaceae)View study →}.

Ramsons is the only other well-quantified allicin source. Its total cysteine sulfoxide content peaks at up to 0.4% of fresh weight in March–April, shortly before flowering, with alliin and isoalliin as the main sulfoxides; its alliinase behaves like garlic’s but differs in substrate specificity ^{44Reference 44Schmitt B et al. · 2005Chemical characterization of Allium ursinum L. depending on harvesting timeView study →}. Garlic itself sits at roughly 0.9% alliin, converting to about 0.4% allicin on complete enzymatic conversion ^{7Reference 7Iberl B et al. · 1990Quantitative determination of allicin and alliin from garlic by HPLCView study →}.

Within A. sativum, cultivar and growing conditions move these numbers substantially, and organosulfur composition correlates with measured antiplatelet activity — so “a clove of garlic” is a considerably less precise dose than it sounds ^{45Reference 45Shang A et al. · 2019ReviewBioactive compounds and biological functions of garlic (Allium sativum L.) (review)View study →}. Commercial products vary even more: a survey of garlic preparations found thiosulfinates released only from cloves and garlic powder products, vinyldithiins and ajoenes only in oil macerates, the diallyl/methyl-allyl/dimethyl sulfide series only in steam-distilled oil products, and no detectable non-ionic sulfur compounds at all in gel-suspended powders or alcohol-aged garlic — with up to a 33-fold range between brands of the same product type ^{8Reference 8Lawson LD et al. · 1991Identification and HPLC quantitation of the sulfides and dialk(en)yl thiosulfinates in commercial garlic productsView study →}.

Major Natural Sources

Source	Part	Precursor chemotype	Allicin-yielding?
Garlic (Allium sativum)	Bulb / clove	Alliin-dominant (~0.9% fresh wt)	Yes — the reference source, ~0.4% yield 7,43Reference 7Iberl B et al. · 1990Quantitative determination of allicin and alliin from garlic by HPLCView study →Reference 43Fritsch RM · 2006Occurrence and taxonomic significance of cysteine sulphoxides in the genus Allium L. (Alliaceae)View study →
Ramsons / wild garlic (A. ursinum)	Leaf, bulb	Triple mix: methiin + alliin + isoalliin, up to 0.4% total	Yes, partially — alliin is one of three 43,44Reference 43Fritsch RM · 2006Occurrence and taxonomic significance of cysteine sulphoxides in the genus Allium L. (Alliaceae)View study →Reference 44Schmitt B et al. · 2005Chemical characterization of Allium ursinum L. depending on harvesting timeView study →
Sand leek (A. scorodoprasum)	Bulb	Alliin-dominant (“garlic type”)	Yes 43Reference 43Fritsch RM · 2006Occurrence and taxonomic significance of cysteine sulphoxides in the genus Allium L. (Alliaceae)View study →
Wild leek (A. obliquum)	Bulb	Alliin-dominant (“garlic type”)	Yes 43Reference 43Fritsch RM · 2006Occurrence and taxonomic significance of cysteine sulphoxides in the genus Allium L. (Alliaceae)View study →
Onion, chive, top onion	Bulb, leaf	Isoalliin-dominant (“onion type”)	No — yields the lachrymatory factor instead 43Reference 43Fritsch RM · 2006Occurrence and taxonomic significance of cysteine sulphoxides in the genus Allium L. (Alliaceae)View study →
Leek, pearl onion (A. ampeloprasum)	Stem, bulb	Methiin / propiin-leaning	Negligible 43Reference 43Fritsch RM · 2006Occurrence and taxonomic significance of cysteine sulphoxides in the genus Allium L. (Alliaceae)View study →
Chinese leek (A. tuberosum)	Leaf	Alliin and isoalliin co-dominant	Partially 43Reference 43Fritsch RM · 2006Occurrence and taxonomic significance of cysteine sulphoxides in the genus Allium L. (Alliaceae)View study →

Discovery & Synthesis

First isolation (1944). Allicin was isolated, named and characterised by Chester J. Cavallito and John Hays Bailey at Winthrop Chemical Company, who identified it as “the antibacterial principle of Allium sativum” and described its physical properties and antibacterial action ^{2Reference 2Cavallito CJ · 1944Allicin, the antibacterial principle of Allium sativumView study →}. Cavallito, Buck and Suter assigned the chemical structure in a companion paper the same year ^{3Reference 3Cavallito CJ et al. · 1944Allicin, the antibacterial principle of Allium sativumView study →}. The name derives directly from the genus Allium.

The precursor (1948–51). The obvious question — where does it come from, given that intact garlic has none? — was answered by Arthur Stoll and Ewald Seebeck at Sandoz, who identified and synthesised alliin and characterised the alliinase that converts it ^{4Reference 4Stoll A · 1951Chemical investigations on alliin, the specific principle of garlicView study →}. This is the work that made allicin’s ephemerality a fact rather than a curiosity.

Total synthesis. Allicin is straightforward to prepare synthetically — typically by oxidation of diallyl disulfide — and synthetic material has been used to establish HPLC standards ^{7Reference 7Iberl B et al. · 1990Quantitative determination of allicin and alliin from garlic by HPLCView study →} and to compare the parent molecule with its transformation products directly ^{51Reference 51Ilić DP et al. · 2015In vitroBiological evaluation of synthesized allicin and its transformation products obtained by microwaves in methanol: antioxidant activity and effect on cell growth (in vitro)View study →}.

Manufacture today. There is essentially no commercial market in isolated allicin, because there is no way to keep it. Commerce runs on the precursor system instead: garlic powder dried carefully enough to preserve alliin and alliinase activity — drying at 60 °C leaves alliin and diallyl thiosulfinate formation untouched, though it degrades isoalliin and slows methyl thiosulfinate formation ^{5Reference 5Lawson LD · 1992Characterization of the formation of allicin and other thiosulfinates from garlicView study →}. Stabilised aqueous allicin extracts do exist for topical and laboratory use ^{24Reference 24Cutler RR · 2004In vitroAntibacterial activity of a new, stable, aqueous extract of allicin against methicillin-resistant Staphylococcus aureus (in vitro)View study →}, and analytical-grade allicin is made synthetically.

Patents: not yet researched.

Commercial Production

The garlic supplement category is best understood as four chemically distinct products that share a plant but not a molecule ^{8,11Reference 8Lawson LD et al. · 1991Identification and HPLC quantitation of the sulfides and dialk(en)yl thiosulfinates in commercial garlic productsView study →Reference 11Lawson LD · 2005Clinical trialComposition, stability, and bioavailability of garlic products used in a clinical trialView study →}:

• Garlic powder tablets/capsules — dried clove; carries alliin plus alliinase, so it generates allicin if the enzyme survives to a favourable pH. This is the only category with real allicin potential, and the only one where enteric coating and dissolution rate matter ^{9,10Reference 9Lawson LD · 2001Low allicin release from garlic supplements: a major problem due to the sensitivities of alliinase activityView study →Reference 10Lawson LD · 2018Clinical trialAllicin bioavailability and bioequivalence from garlic supplements and garlic foods (human volunteers)View study →}.
• Steam-distilled garlic oil — contains the diallyl, methyl allyl and dimethyl sulfide series exclusively; no thiosulfinates at all ^{8Reference 8Lawson LD et al. · 1991Identification and HPLC quantitation of the sulfides and dialk(en)yl thiosulfinates in commercial garlic productsView study →}.
• Oil-macerated garlic — contains the vinyldithiins and ajoene, allicin’s rearrangement products, and only about 20% of the total sulfur compounds of fresh cloves ^{8Reference 8Lawson LD et al. · 1991Identification and HPLC quantitation of the sulfides and dialk(en)yl thiosulfinates in commercial garlic productsView study →}.
• Aged garlic extract — aged in aqueous alcohol; contains no detectable thiosulfinates or non-ionic sulfur compounds, and is standardised instead to the stable, well-absorbed S-allylcysteine ^{8,11Reference 8Lawson LD et al. · 1991Identification and HPLC quantitation of the sulfides and dialk(en)yl thiosulfinates in commercial garlic productsView study →Reference 11Lawson LD · 2005Clinical trialComposition, stability, and bioavailability of garlic products used in a clinical trialView study →}.

Between brands within a single category, total sulfur content varied up to 4-fold for oil macerates and 33-fold for steam-distilled oils ^{8Reference 8Lawson LD et al. · 1991Identification and HPLC quantitation of the sulfides and dialk(en)yl thiosulfinates in commercial garlic productsView study →}. This heterogeneity is the main reason the garlic clinical literature is so hard to synthesise — trials nominally testing “garlic” have often been testing chemically unrelated products.

Toxicity & Safety

Culinary garlic is one of the safest foods in the world, and nothing here contradicts that. The moderate flag applies to the isolated molecule, which is a reactive, non-selective thiol reagent — the same property that makes it antimicrobial makes it cytotoxic without discrimination ^{1,21Reference 1Borlinghaus J et al. · 2014ReviewAllicin: chemistry and biological properties (review)View study →Reference 21Borlinghaus J et al. · 2021ReviewAllicin, the odor of freshly crushed garlic: a review of recent progress in understanding allicin’s effects on cellsView study →}.

Direct Cytotoxicity & Irritation

In mammalian cell lines allicin induces apoptosis at moderate concentrations and outright necrosis at higher ones ^{1,21Reference 1Borlinghaus J et al. · 2014ReviewAllicin: chemistry and biological properties (review)View study →Reference 21Borlinghaus J et al. · 2021ReviewAllicin, the odor of freshly crushed garlic: a review of recent progress in understanding allicin’s effects on cellsView study →}; in the perfused rat liver, concentrations high enough for allicin to pass through unmetabolised also caused considerable hepatocyte injury ^{40Reference 40Egen-Schwind C et al. · 1992AnimalMetabolism of garlic constituents in the isolated perfused rat liverView study →}. Orally, concentrated garlic preparations are gastric irritants, and the Cochrane cold review recorded rash and odour as adverse effects even at supplement doses ^{35Reference 35Lissiman E et al. · 2014Systematic reviewGarlic for the common cold (Cochrane systematic review)View study →}.

Chemical Burns from Topical Raw Garlic

The most concrete real-world harm attached to this molecule. Prolonged skin contact with crushed raw garlic — usually as a folk remedy, occlusive dressing or poultice — produces partial-thickness chemical burns, documented in children ^{48Reference 48Rafaat M · 2000Case reportGarlic burns (case report)View study →} and adults ^{49Reference 49Oberle M et al. · 2016Case reportGarlic burn to the face (case report)View study →}. Occlusion and duration are the risk factors. Crushed raw garlic should not be left on skin.

Contact Allergy

Garlic is a recognised contact allergen, particularly as an occupational dermatitis in cooks and food handlers. The principal sensitiser is diallyl disulfide — an allicin decomposition product — rather than allicin itself; patch-test series confirm meaningful sensitisation rates in exposed populations ^{50Reference 50Bordel-Gómez MT · 2008Sensitivity to diallyl disulfide in a Spanish populationView study →}. People with Allium allergy should avoid concentrated preparations.

Bleeding Risk & Interactions

Garlic’s thiosulfinates inhibit platelet aggregation ^{37Reference 37Lawson LD et al. · 1992Inhibition of whole blood platelet-aggregation by compounds in garlic clove extracts and commercial garlic productsView study →}, and the effect tracks allicin content across preparations ^{13Reference 13Cavagnaro PF et al. · 2007Effect of cooking on garlic (Allium sativum L.) antiplatelet activity and thiosulfinates contentView study →}. The standard clinical caution therefore applies to allicin-generating products in particular: caution with warfarin, antiplatelet agents and in the perioperative period. Notably, a controlled study of aged garlic extract — which contains no allicin — found it did not meaningfully affect warfarin therapy ^{47Reference 47Macan H et al. · 2006Clinical trialAged garlic extract may be safe for patients on warfarin therapy (clinical trial)View study →}, which is consistent with the antiplatelet activity belonging to the thiosulfinate/sulfide fraction rather than to garlic as such.

The best-documented pharmacokinetic interaction is with saquinavir: garlic supplements reduced saquinavir exposure substantially in healthy volunteers, an effect attributed to induction of intestinal metabolism/transport ^{46Reference 46Piscitelli SC et al. · 2002Clinical trialThe effect of garlic supplements on the pharmacokinetics of saquinavir (human volunteers)View study →}. This warrants caution with protease inhibitors and, by extension, other narrow-therapeutic-index substrates of the same routes. No comprehensive CYP-isoform interaction package exists for isolated allicin.

Reproductive & Genotoxicity Scope

No dedicated reproductive- or developmental-toxicity dataset exists for isolated allicin, and no repeat-dose or genotoxicity package has been characterised for the pure compound at the level available for stable garlic constituents. Absence of findings here reflects absence of study, not evidence of safety. Decisions about garlic intake in pregnancy or lactation belong on the garlic herb page, where intake is actually decided.

Safety at a Glance

⚠️ Moderate toxicityBleeding riskDrug interactionsAllergy risk

• Culinary garlic is very safe. Ordinary dietary use has an excellent record, and the reactive molecule is largely destroyed before it goes anywhere.
• Chemical burns from topical raw garlic. Prolonged or occluded skin contact causes partial-thickness burns — documented in children and adults.
• Antiplatelet effect and bleeding risk. Allicin-generating preparations inhibit platelet aggregation; caution with warfarin, antiplatelets and before surgery.
• Not a selective agent. The thiol chemistry that kills microbes also modifies host proteins and is cytotoxic to mammalian cells at comparable concentrations.
• Drug interaction with protease inhibitors. Garlic supplements markedly reduced saquinavir exposure in healthy volunteers.
• Contact allergy. Diallyl disulfide, an allicin decomposition product, is a recognised occupational sensitiser in cooks and food handlers.

Dosage

There is no established dose of allicin, because there is no way to administer it. Every figure below is an allicin potential — what a preparation could generate — or a whole-preparation dose, and none is a personal recommendation.

Two practical notes rather than doses. Crush and let stand before heating — the enzyme needs to meet its substrate before heat denatures it ^{13Reference 13Cavagnaro PF et al. · 2007Effect of cooking on garlic (Allium sativum L.) antiplatelet activity and thiosulfinates contentView study →}. And take an enteric garlic tablet away from a heavy protein meal, since slower gastric emptying holds it in acid and roughly halves delivery ^{10Reference 10Lawson LD · 2018Clinical trialAllicin bioavailability and bioequivalence from garlic supplements and garlic foods (human volunteers)View study →}.

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

References

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