Garlic

Garlic has long been used in Western herbal medicine as a warming circulatory stimulant, antimicrobial and cardiovascular tonic. Traditional uses include respiratory infections, sluggish circulation, intestinal parasites, raised blood fats and weak digestion.

It was also taken during outbreaks of infectious disease as a protective remedy.

The European Medicines Agency recognises two traditional uses: as an aid to preventing atherosclerosis, and to relieve the symptoms of the common cold ^{28Reference 28Committee on Herbal Medicinal Products ( · 2017ReviewEuropean Union herbal monograph on Allium sativum L., bulbus — final, EMA/HMPC/7685/2013 — regulatory monograph. https://www.ema.europa.eu/en/documents/herbal-monograph/final-european-union-herbal-monograph-allium-sativum-l-bulbus_en.pdfView study →}.

In Traditional Chinese Medicine, garlic (大蒜, da suan) is warm, pungent and dispersing. It is used to resolve toxicity, warm the digestion, kill parasites and move stagnation, and is associated with the Lung, Spleen and Stomach channels.

Ayurveda regards garlic (lasuna) as a heating, penetrating herb that stimulates digestion, circulation and vitality. It is used for respiratory congestion, weakness, parasites and disorders of excess Kapha.

Classical texts describe garlic as rejuvenating in small amounts and overstimulating in excess.

Several independent meta-analyses of placebo-controlled trials find a modest fall in blood pressure. A 2015 review of seven trials in people with hypertension found systolic pressure fell by about 6.7 mmHg and diastolic by about 4.8 mmHg against placebo ^{1Reference 1Xiong XJ et al. · 2015Meta-analysisGarlic for hypertension: a systematic review and meta-analysis of randomized controlled trials — systematic reviewView study →}.

A 17-trial meta-analysis found reductions of about 3.75/3.4 mmHg overall. The systolic fall was significant in people with hypertension but not in those with normal blood pressure ^{3Reference 3Wang HP et al. · 2015Meta-analysisEffect of garlic on blood pressure: a meta-analysis — randomised trialsView study →}.

Earlier and updated analyses agree on the direction and on the pattern that the effect is largest when baseline pressure is high. In hypertensive participants the fall reached about 8.4–8.7 mmHg systolic ^{2,4Reference 2Ried K et al. · 2008Meta-analysisEffect of garlic on blood pressure: a systematic review and meta-analysisView study →Reference 4Ried K · 2016Meta-analysisGarlic lowers blood pressure in hypertensive individuals, regulates serum cholesterol, and stimulates immunity: an updated meta-analysis and reviewView study →}.

A 2024 meta-analysis of aged garlic extract that reported benefit only above 1200 mg/day has since been retracted ^{5Reference 5Saadh 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 →}, so no dose threshold for aged extract is established.

Gap: trials are short, preparations and doses vary widely, and heterogeneity between studies remains high; no trial tests hard cardiovascular endpoints.

The lipid evidence is large but mixed. A 26-study analysis found total cholesterol fell by about 0.28 mmol/L and triglycerides by 0.13 mmol/L, with no effect on LDL or HDL ^{7Reference 7Zeng T et al. · 2012Meta-analysisA meta-analysis of randomized, double-blind, placebo-controlled trials for the effects of garlic on serum lipid profilesView study →}. A 21-trial 2024 analysis in people with dyslipidaemia reported larger falls in total cholesterol, LDL and triglycerides, with a slight rise in HDL ^{8Reference 8Du Y et al. · 2024Meta-analysisGarlic consumption can reduce the risk of dyslipidemia: a meta-analysis of randomized controlled trialsView study →}.

A 13-trial meta-analysis found no significant effect on any lipid measure ^{6Reference 6Khoo YSK et al. · 2009Meta-analysisGarlic supplementation and serum cholesterol: a meta-analysisView study →}. An earlier analysis of 13 trials found an overall fall of 0.41 mmol/L, but the six diet-controlled trials with the highest quality scores showed no significant difference from placebo ^{9Reference 9Stevinson C et al. · 2000Meta-analysisGarlic for treating hypercholesterolemiaView study →}.

Garlic powder and aged extract lowered total cholesterol more, and garlic oil lowered triglycerides more ^{7Reference 7Zeng T et al. · 2012Meta-analysisA meta-analysis of randomized, double-blind, placebo-controlled trials for the effects of garlic on serum lipid profilesView study →}. Effects were larger with higher baseline cholesterol and longer treatment ^{7Reference 7Zeng T et al. · 2012Meta-analysisA meta-analysis of randomized, double-blind, placebo-controlled trials for the effects of garlic on serum lipid profilesView study →}.

Gap: the best-designed, diet-controlled trials tend to shrink the effect toward null, so the pooled benefit is sensitive to trial quality and likely small in real-world use.

A four-year randomised, double-blind, placebo-controlled trial of high-dose garlic powder in 152 people measured plaque in the carotid and femoral arteries by ultrasound. Garlic reduced the growth in plaque volume by 5–18%, with slight regression in some cases ^{10Reference 10Koscielny J et al. · 1999RCTThe antiatherosclerotic effect of Allium sativum — randomized, double-blind, placebo-controlled trialView study →}.

A 3-month trial in 56 people with coronary artery disease gave garlic powder tablets (1200 µg allicin each) twice daily alongside standard care ^{11Reference 11Mahdavi-Roshan M et al. · 2013RCTEffect of garlic powder tablet on carotid intima-media thickness in patients with coronary artery disease: a preliminary randomized controlled trialView study →}. Carotid intima-media thickness stayed stable on garlic and increased on placebo, while blood fats did not differ between the groups ^{11Reference 11Mahdavi-Roshan M et al. · 2013RCTEffect of garlic powder tablet on carotid intima-media thickness in patients with coronary artery disease: a preliminary randomized controlled trialView study →}.

Both trials used dried garlic powder and measured artery imaging rather than heart attacks or strokes.

A 2022 systematic review of 12 trials (595 participants) found that six showed garlic reducing platelet aggregation ^{12Reference 12Mollahosseini M et al. · 2022Systematic reviewA systematic review of randomized controlled trials related to the effects of garlic supplementation on platelet aggregationView study →}. Because the trials were few and varied, the authors could not reach a firm conclusion ^{12Reference 12Mollahosseini M et al. · 2022Systematic reviewA systematic review of randomized controlled trials related to the effects of garlic supplementation on platelet aggregationView study →}.

In rabbits, pretreatment with an aqueous garlic extract (500 mg/kg) reduced thromboxane-B2 production and protected against collagen- and arachidonic-acid-induced clotting ^{13Reference 13Ali M et al. · 1990AnimalAntithrombotic activity of garlic: inhibition of thromboxane-B2 synthesis during infusion of arachidonic acid and collagen in rabbits — animal modelView study →}.

The antiplatelet activity tracks allicin and the thiosulfinates ^{14Reference 14Cavagnaro PF et al. · 2007In vitroEffect of cooking on garlic (Allium sativum L.) antiplatelet activity and thiosulfinates content — in vitroView study →}. In laboratory tests, heating uncrushed cloves for six minutes abolished the effect, while crushing before cooking preserved part of it ^{14Reference 14Cavagnaro PF et al. · 2007In vitroEffect of cooking on garlic (Allium sativum L.) antiplatelet activity and thiosulfinates content — in vitroView study →}.

The same activity is the basis of garlic’s bleeding caution around surgery and blood thinners.

The antimicrobial activity of garlic comes from allicin, the thiosulfinate formed when a clove is crushed. Allicin targets thiol-containing proteins and enzymes in microbes and can regulate their virulence genes ^{15Reference 15Choo S et al. · 2020ReviewAntimicrobial properties of allicin used alone or in combination with other medications — reviewView study →}. It also strengthened the effect of antibiotics and antifungals in combination ^{15Reference 15Choo S et al. · 2020ReviewAntimicrobial properties of allicin used alone or in combination with other medications — reviewView study →}.

Garlic’s organosulfur compounds, including ajoene and the allyl sulfides, show bactericidal, antibiofilm and anti-quorum-sensing activity, including against multidrug-resistant strains ^{25Reference 25Bhatwalkar SB et al. · 2021ReviewAntibacterial properties of organosulfur compounds of garlic (Allium sativum) — reviewView study →}.

A crude clove extract disintegrated the membranes of probiotic Bifidobacterium species in the laboratory, much as it does in pathogens ^{16Reference 16Booyens J et al. · 2014In vitroIn vitro antibacterial mechanism of action of crude garlic (Allium sativum) clove extract on selected probiotic Bifidobacterium species — in vitroView study →}.

This activity depends on fresh crushed garlic, since aged extracts and cooked garlic contain little allicin.

Gap: the entire case is in vitro or animal; no controlled human trials show systemic antimicrobial efficacy, and allicin-derived compounds are rapidly metabolised when swallowed ^{27Reference 27Linus Pauling InstituteGarlicView study →}.

A 2017 meta-analysis of nine trials (768 people with type 2 diabetes) found garlic significantly reduced fasting blood glucose, with larger falls at 12 and 24 weeks ^{17Reference 17Wang J et al. · 2017Meta-analysisEffect of garlic supplement in the management of type 2 diabetes mellitus: a meta-analysis of randomized controlled trialsView study →}. Fructosamine and HbA1c also fell, and total cholesterol and LDL improved ^{17Reference 17Wang J et al. · 2017Meta-analysisEffect of garlic supplement in the management of type 2 diabetes mellitus: a meta-analysis of randomized controlled trialsView study →}.

Doses ranged from 0.05 to 1.5 g of garlic or allicin a day ^{17Reference 17Wang J et al. · 2017Meta-analysisEffect of garlic supplement in the management of type 2 diabetes mellitus: a meta-analysis of randomized controlled trialsView study →}. No long-term study has tested whether the effect reduces diabetic complications.

A 2001 double-blind trial randomised 146 people to an allicin-containing supplement or placebo, one capsule daily for 12 weeks. The garlic group had 24 colds against 65 on placebo, and 111 days of illness against 366 ^{19Reference 19Josling P · 2001RCTPreventing the common cold with a garlic supplement: a double-blind, placebo-controlled survey — randomized controlled trialView study →}.

The trial reported faster recovery on garlic ^{19Reference 19Josling P · 2001RCTPreventing the common cold with a garlic supplement: a double-blind, placebo-controlled survey — randomized controlled trialView study →}. The 2014 Cochrane review found the number of days to recovery similar in both groups ^{18Reference 18Lissiman E et al. · 2014Systematic reviewGarlic for the common cold — systematic reviewView study →}. It identified this as the only trial meeting its criteria and judged the evidence insufficient ^{18Reference 18Lissiman E et al. · 2014Systematic reviewGarlic for the common cold — systematic reviewView study →}.

Gap: a single small RCT with subjective self-reported endpoints; no independent replication.

Meta-analyses of mostly case-control studies link high garlic or Allium-vegetable intake with roughly half the risk of gastric cancer (odds ratios about 0.5) ^{20,22Reference 20Kodali RT et al. · 2015Meta-analysisMeta-analysis: does garlic intake reduce risk of gastric cancer? Nutrition and Cancer. https://pubmed.ncbi.nlm.nih.gov/25411831/View study →Reference 22Zhou Y et al. · 2011Meta-analysisConsumption of large amounts of Allium vegetables reduces risk for gastric cancer in a meta-analysis — cohort and case-controlView study →}.

A 2018 meta-analysis of 18 studies reproduced this (odds ratio 0.51), but the link was not significant in prospective studies alone (odds ratio 0.95) ^{21Reference 21Li Z et al. · 2018Meta-analysisThe association of garlic with Helicobacter pylori infection and gastric cancer risk: a systematic review and meta-analysisView study →}. Ten of thirteen studies of garlic and Helicobacter pylori infection found no significant effect ^{21Reference 21Li Z et al. · 2018Meta-analysisThe association of garlic with Helicobacter pylori infection and gastric cancer risk: a systematic review and meta-analysisView study →}.

Gap: case-control designs are prone to recall and selection bias; the prospective-only evidence is null, and this concerns garlic as a dietary constituent, not a therapeutic dose.

Common effects. Breath and body odour are the most common complaints ^{23Reference 23Borrelli F et al. · 2007ReviewGarlic (Allium sativum L.): adverse effects and drug interactions in humans — reviewView study →}. Abdominal pain, bloating, wind, fullness, loss of appetite, headache, dizziness and heavy sweating are also reported ^{28Reference 28Committee on Herbal Medicinal Products ( · 2017ReviewEuropean Union herbal monograph on Allium sativum L., bulbus — final, EMA/HMPC/7685/2013 — regulatory monograph. https://www.ema.europa.eu/en/documents/herbal-monograph/final-european-union-herbal-monograph-allium-sativum-l-bulbus_en.pdfView study →}.

Bleeding. Garlic can alter platelet function, and case reports link it to bleeding ^{23Reference 23Borrelli F et al. · 2007ReviewGarlic (Allium sativum L.): adverse effects and drug interactions in humans — reviewView study →}. Use it with caution alongside oral anticoagulant or antiplatelet therapy, and stop garlic 7 days before surgery because of the risk of bleeding afterwards ^{28Reference 28Committee on Herbal Medicinal Products ( · 2017ReviewEuropean Union herbal monograph on Allium sativum L., bulbus — final, EMA/HMPC/7685/2013 — regulatory monograph. https://www.ema.europa.eu/en/documents/herbal-monograph/final-european-union-herbal-monograph-allium-sativum-l-bulbus_en.pdfView study →}.

Drug interactions. Garlic caplets roughly halved blood levels of the HIV drug saquinavir in volunteers, and garlic is contraindicated with saquinavir/ritonavir ^{24,28,33Reference 24Hu Z et al. · 2005ReviewHerb-drug interactions: a literature reviewView study →Reference 28Committee on Herbal Medicinal Products ( · 2017ReviewEuropean Union herbal monograph on Allium sativum L., bulbus — final, EMA/HMPC/7685/2013 — regulatory monograph. https://www.ema.europa.eu/en/documents/herbal-monograph/final-european-union-herbal-monograph-allium-sativum-l-bulbus_en.pdfView study →Reference 33Piscitelli et al. · 2002Clinical trialThe effect of garlic supplements on the pharmacokinetics of saquinavir — clinical studyView study →}. A likely cause is that garlic raises the gut transporter P-glycoprotein, which pumps drugs back out of the intestinal wall ^{32Reference 32Hajda et al. · 2010Clinical trialGarlic extract induces intestinal P-glycoprotein, but exhibits no effect on intestinal and hepatic CYP3A4 in humans — clinical studyView study →}. It raised the clotting time and INR of warfarin, and caused low blood sugar with chlorpropamide ^{24Reference 24Hu Z et al. · 2005ReviewHerb-drug interactions: a literature reviewView study →}.

Drug metabolism. In healthy volunteers, garlic oil taken for 28 days reduced CYP2E1 activity by 39% but did not change CYP1A2, CYP2D6 or CYP3A4 activity ^{31Reference 31Gurley et al. · 2002Clinical trialCytochrome P450 phenotypic ratios for predicting herb-drug interactions in humans — clinical studyView study →}. A garlic extract likewise left CYP3A4 in the gut and liver unchanged ^{32Reference 32Hajda et al. · 2010Clinical trialGarlic extract induces intestinal P-glycoprotein, but exhibits no effect on intestinal and hepatic CYP3A4 in humans — clinical studyView study →}.

Skin and allergy. Fresh garlic applied to the skin, especially under a dressing, can cause burns ^{23Reference 23Borrelli F et al. · 2007ReviewGarlic (Allium sativum L.): adverse effects and drug interactions in humans — reviewView study →}. Allergic reactions include contact dermatitis, hives, conjunctivitis, rhinitis and, rarely, bronchospasm or anaphylaxis ^{23,28Reference 23Borrelli F et al. · 2007ReviewGarlic (Allium sativum L.): adverse effects and drug interactions in humans — reviewView study →Reference 28Committee on Herbal Medicinal Products ( · 2017ReviewEuropean Union herbal monograph on Allium sativum L., bulbus — final, EMA/HMPC/7685/2013 — regulatory monograph. https://www.ema.europa.eu/en/documents/herbal-monograph/final-european-union-herbal-monograph-allium-sativum-l-bulbus_en.pdfView study →}.

Digestion. Use cautiously with gastric irritation or active digestive inflammation.

Children. Garlic medicines for atherosclerosis are not recommended under 18, and for colds under 12, because of a lack of data ^{28Reference 28Committee on Herbal Medicinal Products ( · 2017ReviewEuropean Union herbal monograph on Allium sativum L., bulbus — final, EMA/HMPC/7685/2013 — regulatory monograph. https://www.ema.europa.eu/en/documents/herbal-monograph/final-european-union-herbal-monograph-allium-sativum-l-bulbus_en.pdfView study →}.

Animal findings. Rats given crude garlic or garlic powder showed reduced sperm production and testosterone, at about twice the highest human dose ^{28Reference 28Committee on Herbal Medicinal Products ( · 2017ReviewEuropean Union herbal monograph on Allium sativum L., bulbus — final, EMA/HMPC/7685/2013 — regulatory monograph. https://www.ema.europa.eu/en/documents/herbal-monograph/final-european-union-herbal-monograph-allium-sativum-l-bulbus_en.pdfView study →}.