Garlic

Garlic

Summary
Binomial	Allium sativum L.
Family	Amaryllidaceae
Common names	garlic, thum or thawm (Arabic), lashuna (Sanskrit), lehsun (Hindi), da suan (Chinese), sir (Persian), ajo (Spanish), ail (French), Knoblauch (German)
Native range	Central Asia (the Tien Shan and Pamir mountain regions); cultivated worldwide since the Bronze Age
Cultivars / varieties	hardneck (Allium sativum var. ophioscorodon) produces a flowering scape and the largest cloves but stores poorly; softneck (var. sativum) is the supermarket type, stores well, produces no scape; elephant garlic is the related Allium ampeloprasum (not true garlic); black garlic is fermented A. sativum, with substantially different (sweeter, less pungent) pharmacology
Parts used	bulb (the clustered cloves); green scape culinary only
Cultivation	hardy bulb planted in autumn for spring harvest in temperate climates; widely cultivated worldwide
Pharmacy
Preparations	raw clove 1-4 g/day (one to four cloves); aged garlic extract 600-1200 mg/day; standardized garlic powder 600-900 mg/day (allicin yield 0.6-1.3% wt/wt); tincture 1:5 in 45% alcohol, 2-4 mL three times daily; topical poultice for fungal skin infection; garlic oil for traditional otitis externa (folk use)
Pregnancy	dietary amounts safe; therapeutic doses of standardized extract caution
Legal status	unscheduled; GRAS for culinary use; sold worldwide as dietary supplement
Pharmacology
Active constituents	alliin (S-allyl-L-cysteine sulfoxide), enzymatically converted by alliinase on crushing to allicin (the principal antimicrobial); allicin degradation products including diallyl disulfide, diallyl trisulfide, ajoene; S-allyl-L-cysteine (the principal aged garlic extract component); fructans (inulin); organoselenium compounds
Mechanism (summary)	allicin and ajoene inhibition of HMG-CoA reductase (modest lipid-lowering); endothelial nitric oxide synthase activation and hydrogen sulfide release (modest blood-pressure-lowering, vasodilation); inhibition of platelet aggregation via thromboxane A2 reduction; broad-spectrum antimicrobial via allicin thiol-disulfide exchange with bacterial enzymes; CYP3A4 induction (the major clinical interaction substrate)

Garlic is the bulb of Allium sativum L., a perennial bulbous member of the Amaryllidaceae cultivated for at least five thousand years and used continuously as both food and medicine from the Bronze Age to the present. The species epithet sativum is the Latin agricultural suffix meaning "cultivated," reflecting that no wild ancestor of true garlic survives; the plant is thought to derive from Allium longicuspis of the Central Asian Tien Shan and Pamir mountains and to have been carried west along the Silk Road and south into the Indian subcontinent before recorded history. Herodotus, writing in the fifth century BCE about the construction of the Great Pyramid at Giza some two thousand years before his own time, reported that the cost of "radishes, onions, and garlic" issued as daily rations to the workmen was inscribed at the pyramid's base at sixteen hundred talents of silver. The Egyptian medical papyrus known as the Codex Ebers, composed around 1550 BCE during the reign of Amenhotep I, prescribes garlic for twenty-two distinct complaints including headache, parasites, weakness, and the bites of vermin. The continuous documented clinical use of garlic across Mediterranean, Islamic, Ayurvedic, Chinese, and modern Western traditions is rivalled in the materia medica only by opium, wine, and the date palm.

History and traditional use

The Egyptian record is the earliest sustained one. The Codex Ebers of approximately 1550 BCE includes garlic in formulations for headache, fatigue, parasites, hypertension (in the form of a recognised disease called "the disease of weariness in the chest"), and the bites of the asp. The pyramid workers' garlic ration recorded by Herodotus is corroborated by the discovery of clay garlic models in Tutankhamun's tomb and by mummified garlic cloves preserved in the dry climate of Egyptian burials. The Greek tradition adopted garlic as both food and medicine; Hippocrates in the fifth century BCE prescribed it for respiratory complaints, parasites, fatigue, and digestive disorders, and Dioscorides in book two of his De Materia Medica catalogued its use for cough, intestinal worms, snake bite, and the suppression of menstruation. Olympic athletes consumed garlic before competition for its reputed strengthening effect; Greek soldiers carried it on campaign.

The Roman tradition continued and expanded the Greek inheritance. Pliny the Elder in book twenty of his Naturalis Historia (first century CE) lists sixty-one separate medicinal applications of garlic, the most of any single plant in his materia medica. Roman soldiers consumed garlic for strength and protection against the diseases of camp life; the plant followed the legions into every province of the empire, naturalizing in soils across Europe and North Africa. The Mediterranean culinary and medicinal use of garlic has continued unbroken from this period to the present.

The Islamic tradition documented garlic as thum or thawm, with a characteristic ambivalence preserved in the hadith literature: medicinal use is encouraged for many indications, but the strong odor on the breath disqualifies a user from attending the mosque immediately after eating raw cloves. The classical Islamic medical tradition codified this practical distinction by recommending cooked or roasted preparations where breath-odor mattered and reserving raw garlic for therapeutic use. Avicenna in book two of his Canon of Medicine (early eleventh century) lists garlic for respiratory complaints, gastrointestinal parasites, snake and scorpion bite, joint pain, and as a strengthener of weak constitutions. The Persian tradition (sir in Farsi) elaborated the same applications and added the cardiovascular indication: the use of garlic for "thickened blood" and chest pain is consistent across the medieval Persian medical texts.

The Ayurvedic tradition documented garlic as lashuna in the Charaka Samhita and Sushruta Samhita (the foundational Ayurvedic texts compiled in the early centuries of the common era). The plant is classed as rasayana (rejuvenative), with the unusual property of containing five of the six classical Ayurvedic tastes (all except sour) and the corresponding multi-dosha activity. Charaka prescribes garlic for vata disorders generally (the cold-and-dry imbalance), for hridroga (heart disease), for udara (abdominal swelling), and for chronic respiratory complaints; the same text contraindicates large doses for pitta-predominant or summer-season use. The Indian culinary tradition treats garlic as both food and medicine, and the religious-dietary objection to garlic in some Vaishnava and Jain communities (on the grounds of its rajasic-tamasic character disturbing the meditative mind) is the inverse-image of the Ayurvedic clinical endorsement: same characterization, different valence.

The Chinese tradition adopted garlic as da suan and treats it as a warming, pungent food-medicine that disperses cold, kills worms, and resolves toxin (jie du). The plant is not part of the formal materia medica of the major TCM classics in the way Glycyrrhiza uralensis or Zingiber officinale are, but it appears across the folk tradition and in modern Chinese herbal pharmacopoeias as a treatment for cold-pattern abdominal pain, parasites, and the early stages of cold-type respiratory infection.

The medieval European tradition continued the Mediterranean inheritance through monastic and folk medicine. Hildegard von Bingen in her Physica of the twelfth century lists garlic for cough, hoarseness, and the "viscous humors" of the chest, with the unusual practical recommendation to consume it raw because cooking diminished its medicinal effect. Nicholas Culpeper in The English Physitian of 1652 wrote that garlic "hath a special virtue against all cold diseases" and prescribed it for cough, expectoration, and as an anthelmintic. The plague-prophylactic legend of the Four Thieves Vinegar, said to have originated during the Marseille plague of 1720 with a group of grave-robbers whose garlic-infused vinegar protected them from infection, sits between folklore and history; the recipe (garlic, sage, rosemary, lavender, thyme, mint, wormwood in vinegar) is preserved in pharmacopoeias of the period as Vinaigre des Quatre Voleurs, and a related formulation was sold in French pharmacies into the twentieth century.

The early twentieth century saw a return of garlic to formal Western medical use during the First World War, when the British Royal Army Medical Corps used garlic juice as a wound antiseptic for the treatment of trench infections in the absence of effective antibiotic alternatives; the historical reports note substantially reduced post-surgical sepsis with garlic-soaked dressings. The isolation of allicin by Cavallito and Bailey at the Sterling-Winthrop Research Institute in 1944 placed garlic's antimicrobial action on a chemical footing and opened the modern phase of garlic pharmacology.

The contemporary controlled-trial literature has substantially supported the cardiovascular indications. Meta-analyses of randomized blood-pressure trials show a modest but consistent reduction in systolic and diastolic blood pressure with garlic preparations standardized for allicin yield, with the largest effect in hypertensive patients; the Stevinson meta-analysis of garlic for hypercholesterolemia found a modest LDL-lowering effect, though more recent trials with higher methodological rigor have produced equivocal results. The Lissiman 2014 Cochrane review of garlic for the common cold found one small trial supportive of a preventive effect, with insufficient data for therapeutic effect on established colds. Observational studies in high-allium-consumption populations (the Iowa Women's Health Study, the Shandong Province cancer cohort) have shown reduced rates of gastric and colorectal cancer with frequent garlic consumption, though randomized prevention trials have not yet established a definitive causal effect: the fifteen-year follow-up of the Shandong garlic intervention trial found no significant reduction in gastric cancer incidence or mortality with garlic supplementation, despite the strong observational signal.

The modern antimicrobial use of garlic has shifted from the historical broad antiseptic role to specific indications. Allicin has documented in vitro activity against Helicobacter pylori, Candida species, methicillin-resistant Staphylococcus aureus, and several other clinically significant pathogens, but the oral bioavailability of allicin is essentially zero (it is metabolized in the gut and on first pass to other thiosulfinates); the in vivo antimicrobial effect of oral garlic depends on aged-garlic-extract components and on the polysulfide degradation products of allicin rather than on allicin itself. Topical and inhalation formulations preserve more of the original allicin and have correspondingly larger antimicrobial effect.

Botany and identification

Allium sativum is a perennial bulbous plant reaching 30 to 90 cm at flowering. The bulb is the medicinal organ, composed of four to twenty individual cloves enclosed in a papery white or purple-tinged outer sheath. The leaves are flat, grass-like, and concentrated at the base; the flowering scape (in hardneck cultivars only) rises in midsummer bearing an umbel of small pink or white flowers that rarely set viable seed (garlic is propagated almost entirely vegetatively from cloves, with the consequence that all garlic of a given cultivar is essentially a clonal lineage). Distinguished from the wild allium relatives by the large compound bulb composed of multiple discrete cloves, from elephant garlic (Allium ampeloprasum, a different species with one to six much larger cloves and a milder flavor) by the smaller clove size and stronger pungency, and from onion (Allium cepa) by the compound rather than single-bulb structure.

Active constituents

The sulfur-containing secondary metabolites are the principal medicinally active components of garlic, and their chemistry is uniquely complex because the active constituents are generated only on tissue damage. The intact clove contains alliin (S-allyl-L-cysteine sulfoxide), a stable, odorless, water-soluble amino acid derivative, stored in the cytoplasm of the clove cells; in a separate cellular compartment the enzyme alliinase is sequestered, ready to act on alliin only when crushing, cutting, or chewing breaks the cellular membranes and brings the two together. Within seconds of tissue damage alliinase converts alliin to allicin (diallyl thiosulfinate), the principal antimicrobial and the source of garlic's characteristic odor. Allicin is itself unstable; over minutes to hours it decomposes to diallyl disulfide, diallyl trisulfide, ajoene, and several vinyldithiins, each with its own pharmacology. The clinical implication is that the form and processing of garlic determine which sulfur compounds reach the patient: raw crushed garlic delivers transient allicin, dried garlic powder delivers a slower-release allicin (provided alliinase is preserved by enteric coating, which is the basis of the standardized garlic powder preparation), and aged garlic extract delivers very little allicin but substantial quantities of the stable, water-soluble S-allyl-L-cysteine produced by the long fermentation process.

The non-sulfur fraction includes fructans (predominantly inulin, contributing the prebiotic and mild laxative effect at high doses), organoselenium compounds (selenium uptake by garlic is substantially greater than for most cultivated plants, and selenium-enriched garlic is marketed for cancer chemoprevention research), saponins, phenolics, and trace alkaloids.

Preparations

Multiple preparation forms are in current use, each delivering a different sulfur-compound profile: the raw clove (one to four cloves daily, chewed or finely crushed and added to food shortly before consumption) delivers transient high-dose allicin; the standardized garlic powder tablet or capsule (typically 600 to 900 mg daily of dried garlic standardized to 0.6 to 1.3 percent allicin yield, enteric-coated to preserve alliinase) is the form most used in cardiovascular clinical trials; aged garlic extract (600 to 1200 mg daily of the extract produced by twenty-month aging in dilute alcohol) delivers S-allyl-L-cysteine and is the preparation with the most evidence for cardiovascular and immunomodulatory indications; garlic oil (the steam-distilled essential oil, rich in diallyl disulfide and ajoene) is used at 5 to 10 mg daily; tincture (1:5 in 45 percent alcohol, 2 to 4 mL three times daily) is the traditional Western herbalist's form. Topical preparations include the poultice (crushed clove applied directly to fungal skin lesions, with caution against burns on prolonged contact) and the traditional garlic oil (cloves macerated in olive oil) for folk treatment of otitis externa.

Pharmacokinetics

Allicin is rapidly absorbed but extensively metabolized on first pass to diallyl sulfide, diallyl disulfide, and other thiosulfinate degradation products; intact allicin reaches the systemic circulation in negligible quantities after oral administration. The sulfur-compound metabolites are excreted partly via the lungs (the basis of the persistent garlic breath after consumption, which can continue for 24 to 48 hours) and partly via the urine and skin. S-allyl-L-cysteine is the principal stable garlic-derived compound reaching systemic circulation after aged garlic extract; it has a half-life of approximately 10 hours and reaches steady-state concentrations after several days of regular dosing.

Pharmacodynamics

Allicin and ajoene inhibit HMG-CoA reductase in vitro, providing the proposed mechanism for the modest LDL-lowering effect of garlic supplementation. Allicin releases hydrogen sulfide (H2S) on reaction with vascular endothelial thiols, contributing to vasodilation and the modest blood-pressure-lowering effect. Allicin inhibits platelet aggregation through reduction of thromboxane A2 synthesis and through direct thiol-disulfide exchange with platelet membrane proteins; this is the mechanistic basis of the bleeding-risk interaction profile. The broad-spectrum antimicrobial effect of allicin is mediated by thiol-disulfide exchange with bacterial enzymes (most notably the cysteine-containing enzymes of central metabolism), producing a mechanism distinct from that of all major antibiotic classes and accounting for activity against multi-resistant organisms. CYP3A4 induction by aged garlic extract and high-dose standardized preparations is the basis of the clinical interactions with calcineurin inhibitors and antiretroviral protease inhibitors.

Experience

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Problems

Mild hypertension (adjunctive to or in place of pharmaceutical antihypertensive medicine in early or borderline cases). Modest hypercholesterolemia (adjunctive; not a replacement for statins in established cardiovascular disease). Atherosclerosis prevention (the observational and modest-trial evidence base). Common cold prevention (Cochrane-supported, single trial). Gastrointestinal parasites (the historical anthelmintic indication; modern evidence chiefly in vitro). Helicobacter pylori adjunctive treatment. Mild upper respiratory infection. Tinea pedis and onychomycosis (topical and oral). Diabetes mellitus type 2 (adjunctive; modest glycemic effect). Folk indications: otitis externa (garlic oil), insect bite, snake bite (historical, no modern evidence).

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Titration and dosing

Raw clove: 1 to 4 g daily (one medium clove is approximately 3 g). Standardized garlic powder: 600 to 900 mg daily, enteric-coated, providing approximately 5 to 12 mg allicin yield. Aged garlic extract: 600 to 1200 mg daily for cardiovascular indication, up to 7200 mg daily in some research protocols. Garlic oil: 5 to 10 mg daily. Tincture 1:5 in 45% alcohol: 2 to 4 mL three times daily. Topical poultice: applied to the lesion for 10 to 20 minutes, removed before burn develops on healthy skin.

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Effects

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Interactions

clinically significant bleeding risk with warfarin, direct oral anticoagulants, and antiplatelet agents; CYP3A4 induction reduces saquinavir, ritonavir, cyclosporine, and tacrolimus plasma concentrations; preoperative discontinuation 7-10 days before elective surgery is the standard recommendation

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The clinically significant interactions of garlic are dominated by the bleeding-risk and CYP3A4-induction categories. The bleeding risk of garlic combined with warfarin is documented in multiple case reports of supratherapeutic INR and bleeding events; the recommendation is to avoid therapeutic-dose garlic supplementation in warfarin-treated patients, with culinary doses typically considered safe. The same caution applies to the direct oral anticoagulants (apixaban, rivaroxaban, edoxaban, dabigatran), with the additional consideration that the DOAC plasma concentrations may be subject to CYP3A4 induction (apixaban, rivaroxaban) or P-glycoprotein induction (dabigatran, edoxaban) by garlic compounds; the combined effect is unpredictable and the conservative recommendation is to avoid the combination. Antiplatelet medicine (aspirin, clopidogrel, prasugrel, ticagrelor) combined with therapeutic-dose garlic produces additive antiplatelet effect; the clinical consequence is increased bleeding risk, particularly perioperative or in patients with thrombocytopenia or other bleeding diatheses.

CYP3A4 induction by garlic is the basis of the documented interactions with the HIV protease inhibitors (saquinavir, ritonavir), with the calcineurin inhibitors used in transplant medicine (cyclosporine, tacrolimus), and with several other CYP3A4 substrates with narrow therapeutic windows. Reductions of 30 to 50 percent in plasma saquinavir concentrations have been reported with concurrent garlic supplementation, with the clinical risk of antiretroviral failure and emergence of resistance. Similar magnitude reductions in cyclosporine plasma concentrations have been observed; the combination is contraindicated in transplant patients.

The preoperative discontinuation recommendation is seven to ten days before elective surgery for therapeutic-dose garlic supplementation, on the basis of the antiplatelet effect duration and platelet turnover; culinary use does not require preoperative discontinuation.

Pregnancy and lactation

Dietary amounts of garlic are considered safe in pregnancy and lactation, with a long historical record of routine pregnancy consumption in essentially all garlic-using cultures. The flavor of garlic transfers to amniotic fluid and breast milk, and infants exposed to garlic in utero or via maternal lactation show distinct flavor preferences for garlic-containing foods after weaning; some clinical reports note prolonged breastfeeding sessions on days of maternal garlic consumption, with mixed acceptance from mothers. Therapeutic doses of standardized garlic preparations have not been formally studied in pregnancy; the conservative recommendation is to limit pregnant patients to culinary doses, particularly in the third trimester where the antiplatelet effect could theoretically increase peripartum bleeding risk.

Monitoring

For patients on warfarin who choose to consume garlic at culinary doses, INR monitoring at the initiation of regular consumption and again at three to four weeks is the conservative practice; therapeutic-dose supplementation is best avoided rather than monitored. For transplant patients on calcineurin inhibitors, plasma-level monitoring at any change in garlic intake (initiation, discontinuation, or substantial change in quantity) is recommended.

Patient counseling

Patients should be counseled that culinary use of garlic is safe for almost all patients (with the exceptions of those with established Allium allergy, severe Alliaceae cross-reactivity, or active gastrointestinal ulceration where the irritant effect of raw garlic may delay healing). The breath and body odor produced by garlic consumption derives from sulfur metabolites excreted via the lungs and skin and can persist for 24 to 48 hours; chewing parsley, mint, or anise after consumption modestly reduces the breath effect but does not affect the systemic odor. Raw garlic applied directly to skin can produce contact burns, particularly in occluded or prolonged-contact applications; the traditional topical poultice should be limited to 10 to 20 minutes of contact and inspected at intervals. Improperly preserved garlic-in-oil at room temperature is a documented source of Clostridium botulinum growth, with several documented cases of botulism from home-prepared garlic-in-oil; commercially prepared products are acidified or refrigerated to prevent this, and home preparation should follow the same controls. Patients planning elective surgery should be counseled to discontinue therapeutic-dose garlic supplementation seven to ten days before surgery.

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See also

Onion, Leek, Ginger, Turmeric, Cayenne, Hawthorn, Ginkgo