Garlic

Scientific Name(s): Allium sativum L.
Common Name(s): Allium, Camphor of the poor, Da-suan, Garlic, La-suan, Nectar of the gods, Poor man's treacle, Rustic treacle, Stinking rose

Medically reviewed by Drugs.com. Last updated on Dec 19, 2022.

Clinical Overview

Use

Garlic has been investigated for its effects in cancer, diabetes, dyslipidemia, hypertension, ischemic heart disease, heart failure, liver disease, osteoarthritis, and peripheral vascular disease, among other conditions. Additionally, garlic's antimicrobial and antiplatelet effects have been evaluated. Most evidence supporting the use of garlic is for dyslipidemia and hypertension, although evidence shows equivocal or mixed results, partially due to the difficulty in developing a true placebo, as well as poor methodology and a lack of standardization of preparation.

Dosing

Suggested average daily doses include 2 to 5 g of fresh raw garlic; 0.4 to 1.2 g of dried garlic powder; 2 to 5 mg of garlic oil; 300 to 1,000 mg of garlic extract (as solid material); and 2,400 mg/day of aged garlic extract (liquid).

In a meta-analysis evaluating effects of garlic on blood pressure, doses of garlic powder ranged from 300 to 2,400 mg/day for 2 to 24 weeks.

Contraindications

Garlic is contraindicated in individuals with known allergy to garlic and its constituents.

Pregnancy/Lactation

Garlic may be used in pregnancy and lactation with caution. Consumption by breastfeeding mothers may impact the infant's behavior during breastfeeding, causing prolonged attachment to the breast and increased sucking. Anecdotal evidence suggests that maternal consumption of garlic may result in colic; however, evidence is conflicting.

Interactions

Conflicting evidence exists regarding garlic's impact on the CYP-450 system, namely CYP1A2, 2D6, and 3A4. Garlic's interactions are likely mediated through its impact on P-glycoprotein rather than the CYP-450 systems.

Garlic may enhance the adverse effects of antiplatelet agents, anticoagulants, and thrombolytics, increasing the risk for bleeding. Garlic may enhance the effects of antidiabetes drugs such as metformin, resulting in lowered blood glucose levels. Additionally, garlic may decrease the concentrations of protease inhibitors, reducing their efficacy.

Adverse Reactions

Body odor and malodorous breath are the most common complaints after garlic ingestion. Mild GI adverse reactions (eg, bloating, flatulence, nausea) have been commonly reported with use. Burns have been documented with topical use. Ingestion of large amounts may increase the risk of postoperative and spontaneous bleeding. Allergy, asthma, pneumonia, contact dermatitis, and anaphylaxis have been reported.

Toxicology

No data.

Scientific Family

• Liliaceae (lily)

Botany

Garlic is a perennial bulb with a tall, erect flowering stem that grows up to 1.2 m in height. The leaf blade is flat, linear, solid, and approximately 1.25 to 2.5 cm wide, with an acute apex. The plant may produce pink to purple flowers that bloom from July to September in the Northern Hemisphere. The bulb is odiferous and contains outer layers of thin sheathing leaves surrounding an inner sheath that encloses the clove. The bulb often contains 10 to 20 cloves that are asymmetric in shape, except for those closest to the center.Bayan 2014, Butt 2009, WHO 1999

History

The genus Allium comes from the Celtic word for "burning" or "smarting." Early references to garlic were found in a collection of Zoroastrian holy writings known as Avesta that were likely compiled in the sixth century BC. During the early Olympics in Greece, garlic was given to athletes to improve stamina.Bayan 2014 Hippocrates treated chest and stomach pains with garlic.Strickler 2019 During the Middle Ages, physicians prescribed garlic to cure deafness, and gravediggers used it to ward off the Bubonic plague. American Indians used garlic as a remedy for earache, flatulence, and scurvy. In World War II, garlic extracts were used to disinfect wounds. During the 1800s, physicians routinely prescribed garlic inhalation for the treatment of tuberculosis, and raw garlic applied as a poultice continues to be used by naturopaths today.Butt 2009, Rao 2009, Strickler 2019, WHO 1999

Chemistry

Fresh garlic is composed of approximately 63% water, 28% carbohydrates (fructans), 2.3% organosulfar compounds, 2% proteins (alliinase), 1.5% fiber, and 1.2% free amino acids (arginine). Garlic contains the highest sulfur content of any member of the genus Allium. The trace elements germanium and selenium have been found in detectable quantities and are postulated to play a role in garlic's antitumor effect.

Garlic contains approximately 0.5% of a volatile oil composed of sulfur-containing compounds (diallyldisulfide, diallyltrisulfide, methylallyltrisulfide), vinyldithiins, and ajoenes. The bulbs contain alliin (S-allyl-L-cysteine sulfoxide), an odorless, colorless, sulfur-containing amino acid that has no pharmacologic activity of its own. Upon grinding the bulb, the enzyme allinase is released, resulting in conversion of alliin to 2-propenesulfenic acid, which dimerizes to form allicin and further decomposes to diallyl sulfide, -disulfide, -trisulfide, and sulfur dioxide. Allicin gives the pungent, characteristic odor to crushed garlic and is believed to be responsible for some of the pharmacologic activity of the plant.Bayan 2014, Butt 2009, Kimura 2017, WHO 1999

Black garlic is fresh garlic that has been fermented at a high temperature and under high humidity. It has less flavor due to a smaller content of allicin.Kimura 2017

Uses and Pharmacology

Anti-inflammatory effects

Animal data

In a murine model, garlic oil and diallyl disulfide (the organosulfur component of garlic) prevented cigarette smoke–induced airway inflammation. Specifically, it inhibited the increase in neutrophils and decreased the production of interleukin 1-beta (IL-1beta), IL-6, and tumor necrosis factor alpha.(Ko 2018) In a rat model of acute kidney injury, intragastric aged black garlic extract was found to prevent renal dysfunction and structural changes; these effects were attributed to garlic's antioxidant and anti-inflammatory properties.(Lee 2019)

Clinical data

In a meta-analysis of 9 randomized trials (N=363), garlic intake significantly decreased C-reactive protein (CRP) concentrations by 0.8 mg/dL (95% CI, –1.5 to –0.1; P=0.02). Garlic doses of 1,200 mg/day or more significantly decreased CRP by 0.82 mg/dL (95% CI, –1.02 to –0.62; P<0.001). In studies with a baseline CRP of 2 mg/dL or greater, garlic reduced CRP by 2.44 mg/dL (95% CI, –4.02 to –0.87; P=0.002).(Taghizadeh 2019)

Garlic extract 400 mg twice daily for 8 weeks reduced inflammation as measured by IL-6, CRP, and erythrocyte sedimentation rate (ESR) in 42 patients undergoing peritoneal dialysis.(Zare 2019) A systematic review of randomized controlled trial data identified 6 studies overall that met inclusion criteria and investigated oral spice supplementation for rheumatoid arthritis. The one study that assessed garlic was double-blinded and administered garlic tablets 1 g (equivalent to 2.5 g fresh garlic/half clove) or placebo for 8 weeks in 70 adults currently treated with conventional synthetic disease-modifying antirhematic drugs. Compared to placebo, garlic significantly improved 4 of 7 outcome measures (P=0.018 to P<0.001), including disease activity ESR score, tender joint count, pain, and C-reactive protein.(Letarouilly 2020)

Antimicrobial activity

The antibacterial effects of garlic are mostly attributed to the allicin component. Garlic has demonstrated activity against a wide range of gram-positive bacteria, gram-negative bacteria, and fungal species.(Bayan 2014)

Animal and in vitro data

Garlic inhibited Helicobacter pylori in vitro,(Liu 2010) and has demonstrated inhibitory activity against Pseudomonas aeruginosa and Candida tropicalis in vitro and in animal models.(Diba 2018, Smyth 2010) Garlic clove powder reduced Clostridioides difficile toxin activity (P≤0.047).(Roshan 2018) In an in vitro study, several thioallyl compounds from garlic demonstrated activity against Giardia duodenalis.(Argüello-García 2018) Allicin 2 mg/kg exerts antimycobacterial activity both in vitro and in vivo.(Dwivedi 2018)

In a murine model of hydatid cysts, a methanolic extract of A. sativum given for 30 days reduced the number of hydatid cysts as well as the size and weight of the largest cyst.(Haji Mohammadi 2018)

Clinical data

A double-blind, randomized controlled trial evaluated effects of garlic on vaginal Candida counts during the second half of the menstrual cycle in 63 asymptomatic women colonized with Candida spp. Women were treated with 2.1 g/day of garlic powder (19.2 g/day of allicin) or placebo for 14 days. There was a positive trend showing more women in the garlic group reported improvement in vaginitis symptoms, but no statistically significant difference in cases of vulvovaginal candidiasis was observed. More adverse effects, mostly GI symptoms such as gastric pain and nausea, were reported in the garlic group (P<0.01).(Watson 2014)

A case report describes a 25-year-old woman with a vaginal infection caused by Ureaplasma spp. that was unsuccessfully treated with antibiotics. At a family member's recommendation, she cut a garlic clove and inserted it vaginally for one dose. Her gynecologist reported the infection was cured a few days later.(Bekut 2018)

In a Cochrane systematic review of data from randomized controlled trials comparing garlic with placebo for common cold prevention and treatment, only one 12-week study (N=146) was identified as meeting inclusion criteria. Results showed significantly fewer cold episodes and days of illness with garlic compared with placebo, but similar recovery times were reported between the 2 groups. Reviewers were unable to draw firm conclusions based on evidence from one study.(Lissiman 2014)

Limited clinical studies evaluating effects of garlic in cystic fibrosis found no effect on Pseudomonas aeruginosa infection or on pulmonary function.(Hurley 2010, Smyth 2010)

In a study of 60 children 9 to 12 years of age, a 2.5% garlic extract mouthwash (10 mL) used nightly for 15 days reduced salivary Streptococcus mutans counts and prevented buildup of plaque; effects were comparable to mouthwashes containing chlorhexidine gluconate or another natural ingredient, triphala. While chlorhexidine was considered the superior mouthwash due to better active and passive action on salivary S. mutans, the authors concluded that garlic mouthwash is a good option for a natural alternative; further studies are needed.(Padiyar 2018)

Based on data from 4,521 healthy participants enrolled in 20 randomized controlled trials (including 1 study with aged garlic extract), meta-analyses demonstrated that flavonoid-containing supplements were safe and effective in preventing acute respiratory tract infections (ARTIs) compared to controls with a relative risk (RR) of 0.81 (95% CI, 0.74 to 0.89; P<0.001) and low heterogeneity. A reduction in mean ARTI sick days was also observed with the supplements; however, heterogeneity was significant (weighted mean difference [WMD], −0.56; 95% CI, −1.04 to −0.08; P=0.021). In subgroup analysis, significance in mean ARTI sick days was retained with flavonoid mixtures (as seen with aged garlic extract products) but not with use of single flavonoids (ie, quercetin, catechin). Pooled results from 16 of the trials indicated that adverse reactions were not increased in the flavonoid supplement groups compared to controls.(Yao 2022)

Antioxidant effects

Animal data

Antioxidant activity of garlic and aged black garlic extract has been demonstrated in animal models.(Lee 2019, Pocobelli 2010)

Clinical data

A systematic review and meta-analysis identified 7 randomized, controlled trials (N=317) that investigated the effects of garlic, garlic oil, or aged garlic extract on serum oxidative capacity. Overall, the total antioxidant capacity (TAC) and malondialdehyde levels improved significantly with garlic supplementation (P<0.001 and P=0.01, respectively). However, significant heterogeneity was evident for both outcomes. Female gender resolved heterogeneity in the TAC results. Daily doses studied ranged from 12.3 to 2,400 mg/day given for 2 to 16 weeks in patients with a mean age of approximately 17 to 61 years across a range of clinical scenarios (ie, healthy, type 2 diabetes, pregnant, postmenopausal osteoporosis).(Moosavian 2020)

Antiplatelet effects

Studies evaluating effects of garlic on platelet aggregation have produced inconsistent results, possibly due to variations in study design and in garlic preparations used.

The proposed mechanism for garlic's inhibition of platelet function is via methylallyltrisulfide interference with thromboxane synthesis. Ajoene may also contribute to the observed effects.(Ariga 1981, Boullin 1981, Makheia 1979, McCaleb 1993)

In vitro data

Garlic has demonstrated antiplatelet activity in an in vitro model.(Chen 2019)

Clinical data

In a study of healthy subjects consuming garlic cloves (100 to 150 mg/kg), platelets showed complete inhibition to aggregation induced by 5-hydroxytryptamine.(Boullin 1981) Other studies have shown that ingestion of aged garlic extract can inhibit some platelet functions important for initiating thromboembolic events in arterial circulation.(Steiner 1998) However, in a small study of 18 patients, ingestion of 4.2 g of raw garlic over 1 week did not impair platelet function.(Scharbert 2007) In a double-blind, placebo-controlled study of 14 men, garlic did not affect platelet aggregation.(Morris 1995)

Case studies evaluating a direct causal relationship between garlic consumption and warfarin-related bleeding are lacking. However, there are reports of platelet aggregation inhibition and increased bleeding times in individuals ingesting garlic.(Gravas 2010, Heck 2000, Hu 2005, Mohammed Abdul 2008)

Cancer

Animal and in vitro data

In an in vitro study, analogs of allicin exerted cytotoxic activity against breast cancer cells, with less toxic effects on noncancerous cells.(Bhaumik 2019) In vitro models demonstrated antiproliferative effects of garlic against gastric, colorectal, and small cell lung cancer cells.(Miraghajani 2018) In vitro and in vivo studies have demonstrated that garlic suppresses tumor growth and induces apoptosis in prostate cancer cells.(Yedjou 2019)

Clinical data

Evidence suggests an inverse correlation between garlic intake and incidence of gastric cancers and prostate cancer.(Li 2018, Salem 2011) However, a systematic review that identified 1 medium-quality randomized controlled trial and 13 high-quality cohort studies reported equivocal results for garlic intake and cancer risk (breast, colorectal, hematologic, lung, prostate, stomach), including an increased risk of colorectal cancer reported by 1 study.(Lee 2021) Other reviews, including data from the European Prospective Investigation into Cancer and Nutrition (EPIC) study, found no credible evidence supporting use of garlic for prevention of gastric, breast, lung, and endometrial cancers, and very limited and weak evidence for use in reducing the risk of colon, prostate, esophageal/larynx, ovarian, or renal cancers.(González 2011, Kim 2009, Pocobelli 2010, Powolny 2008, Rivlin 2009, You 2006) A case-control study from China suggested that consumption of raw garlic reduces the risk of lung cancer.(Jin 2013)

Diabetes

Animal data

In a meta-analysis of animal studies, no effects of garlic extract on glucose levels were observed; however, results of studies evaluating individual chemical constituents extracted from garlic suggest effectiveness in lowering glucose levels.(Kook 2009) Garlic exerted a hypoglycemic effect in a murine model of type 1 diabetes.(Behrouj 2018) In another murine model of diabetes, allicin, through reductions in oxidative stress and proinflammatory cytokines, delayed the progression of diabetic nephropathy.(Arellano Buendía 2018)

Clinical data

A systematic review and meta-analysis of 33 studies (N=1,273) evaluated the effect of garlic compared to placebo on lipid profiles and blood glucose in patients with diabetes. The group receiving garlic showed significant reductions in total cholesterol (–16.87 mg/dL; 95% CI, –21.01 to –12.73), low-density lipoprotein (LDL) cholesterol (–9.65 mg/dL; 95% CI, –15.07 to –4.23), and triglycerides (–12.44 mg/dL; 95% CI, –18.19 to –6.69), and a significant increase in high-density lipoprotein (HDL) (3.19 mg/dL; 95% CI, 1.85 to 4.53) (P=0.001 for all comparisons to placebo). Fasting blood sugar was also significantly decreased (10.9 mg/dL; 95% CI, –16.4 to –5.4; P=0.001).(Shabani 2019) In another meta-analysis of 9 studies (N=768) evaluating the efficacy and safety of garlic supplementation in the management of type 2 diabetes, garlic significantly reduced fasting blood glucose levels within 1 to 2 weeks (standardized mean difference [SMD], –1.61; 95% CI, –2.89 to –0.32), at 3 to 4 weeks (SMD, –2.87; 95% CI, –4.74 to –1), at 12 weeks (SMD, –9.57; 95% CI, –12.39 to –6.75), and at 24 weeks (SMD, –21.02; 95% CI, –32.47 to –9.57). Garlic was also associated with improvements in lipid parameters.(Wang 2017)

Dyslipidemia

Garlic's ability to lower cholesterol levels may result from the inhibition of hydroxymethyl glutaryl coenzyme A (HMG-CoA) reductase, the rate-lowering step in cholesterol synthesis. Garlic also enhances catabolism of fatty acids and triglycerides, and saponins could slow the absorption of dietary cholesterol from the GI tract.(Jain 2018)

Garlic cannot be recommended for the management of hyperlipidemia, based on data from reviews and meta-analyses of clinical trials.(Huang 2011, Khoo 2009, Ong 2008, Reinhart 2009, Stevinson 2000) Results have varied, possibly due to differences in garlic preparations used and patient populations.

Animal data

In animal models, garlic extracts have reduced triglyceride levels.(Mollazadeh 2019) Garlic administration also prevented neointima formation and the buildup of cholesterol, triglycerides, collagen, and phospholipids in cholesterol-fed rabbits.(Sobenin 2016)

Clinical data

Clinical data regarding the impact of garlic supplementation on cholesterol parameters are mixed, with most data suggesting a reduction in total cholesterol and varying effects on other lipid parameters.

A 2013 updated meta-analysis evaluated data from 39 studies (N=2,298 adults) of at least 2 weeks' duration and employing placebo controls, with garlic as the sole active treatment (administered mainly as tablets standardized to contain from 600 to 2,500 mcg of allicin or 1,000 mcg/g of S-allylcysteine); patient baseline total serum cholesterol levels were greater than 200 mg/dL. Results showed a significant reduction in total serum cholesterol (17±6 mg/dL) and an insignificant reduction in LDL (by 9±6 mg/dL). Prior systematic reviews and meta-analyses also found only modest reductions in total cholesterol, and no effect on LDL or HDL cholesterol.(Ried 2013) In a meta-analysis of 29 trials through November 2007, total cholesterol was statistically significantly decreased by 0.19 mmol/L (95% CI, –0.33 to –0.06) or 7.35 mg/dL , with no effect on LDL or HDL compared to placebo(Reinhart 2009); another meta-analysis of 13 trials through 2008 found no benefit over placebo.(Khoo 2009)

In 2018, a meta-analysis assessing the effect of garlic on cholesterol parameters included 14 studies from 1981 to 2016. Improvements in total cholesterol (SMD, –1.26; 95% CI, –1.86 to –0.66), LDL cholesterol (SMD, –1.07; 95% CI, –1.67 to –0.47), and HDL cholesterol (SMD, 0.5; 95% CI, 0.06 to 0.94) were statistically significant following garlic administration (aged black garlic, garlic oil, and garlic powder doses ranging from 0.3 to 20 g/day) compared to control. However, no statistical difference was found in triglyceride levels between the 2 groups (SMD, –0.16; 95% CI, –0.87 to 0.55).(Sun 2018)

A 3-month, randomized, placebo-controlled, single-blind study evaluated the effects of garlic powder supplementation (as an adjunct to conventional medical treatment) on plasma lipids, lipoproteins, and carotid intima-media thickness (CIMT) to determine cardiovascular risk and progression of atherosclerosis in 56 patients with coronary artery disease. Those randomized to receive twice-daily garlic powder tablets (1,200 mcg of allicin per tablet) experienced minor reductions in CIMT from baseline (by 0.009±0.007 mm), whereas CIMT in the placebo group increased 0.04±0.01 mm. The 3-month mean CIMT difference from baseline was significantly different between the 2 groups (P<0.001). No significant changes were noted between groups for plasma lipids or lipoproteins.(Mahdavi-Roshan 2013) In a double-blind, randomized controlled trial (N=55), administration of aged garlic extract (2,400 mg/day) for 1 year significantly reduced low-attenuation plaque in the coronary arteries of adults with metabolic syndrome compared to placebo. Total plaque volume, dense calcium, and noncalcified plaque were not significantly affected.(Matsumoto 2016) In a European cohort of 104 asymptomatic patients at high-risk of cardiovascular disease, 1-year administration of aged garlic extract (2,400 mg/day) led to a lower rate of coronary artery calcium (CAC) progression than placebo (20% vs 28%) with an adjusted odds ratio of 2.95 (95% confidence interval [CI], 1.05 to 8.27; P=0.04) in a double-blind, randomized, controlled trial. Similarly, probabilities were statistically significantly higher for patients in the aged garlic group to have lower plasma glucose (P=0.034), interleukin-6 (P=0.049), and systolic blood pressure (mean change, −8 mm Hg, P=0.027).(Wlosinska 2020)

A total of 169 patients with moderate hypercholesterolemia (LDL,130 to 190 mg/dL) completed a 6-month placebo-controlled study that assessed raw garlic versus 2 commercial garlic tablet products and their effects on plasma lipids. No statistically or clinically (10 mg/dL change) significant changes were found between groups in LDL, HDL, triglycerides, or total cholesterol:HDL ratio. Additionally, no significant difference was found in the subgroup of patients with LDL levels above the median at baseline. Patients taking raw garlic experienced more adverse events with body and breath odor reported commonly (57%); meanwhile flatulence was reported "often" or "almost always" with raw garlic (6%), Garlicin (8.5%), Kyolic (8.3%), and placebo (2%). Rash, heartburn, and mouth ulcers occurred rarely overall (1 patient each).(Gardner 2007)

A systematic review and meta-analysis of randomized controlled trials (published through March 10, 2015) investigated the effects of garlic preparations on lipoprotein(a), with 6 trials meeting inclusion criteria (N=286). In 5 of the 6 studies, mean patient age ranged from 53 to 60 years and patients were mildly hypercholesterolemic and/or at moderate to high risk for coronary artery disease (CAD); 1 study enrolled patients 8 to 18 years of age with a positive family history. The majority of studies used a garlic extract at a dosage of 300 mg 3 times daily; one study used 250 mg of aged garlic administered once daily. Duration of supplementation ranged from 8 weeks to 12 months. Although the meta-analysis did not show a significant change in plasma lipoprotein(a) levels after garlic consumption, subanalysis of trials based on duration of therapy identified a significant improvement in lipoprotein(a) in trials lasting longer than 12 weeks (P<0.001). This result was supported by meta-regression analysis. Lipid indices showing significant improvement with garlic supplementation were HDL cholesterol (P=0.003) and plasma triacylglycerols (P=0.021). No serious adverse effects were reported with garlic treatment in any of the trials.(Sahebkar 2016) In another systematic review that focused on randomized controlled trials that enrolled patients with CAD and lipid profiles as their primary outcome, a meta-analysis of pooled data from the 6 included trails (N=263) revealed a significant reduction in total cholesterol (weighted mean difference [WMD], −16.32 mg/dL; P=0.032, moderate heterogeneity) with garlic supplementation compared to placebo. No other lipid parameters were significantly affected. Subgroup analysis showed that garlic capsules led to higher HDL levels than other types of interventions (ie, aged garlic capsules, garlic powder tablets) with a WMD 8.09 mg/dL (95% CI, 2.09 to 14.10; substantial heterogeneity). Duration of supplementation ranged from 2 to 48 weeks, the age of participants ranged from 15 to 62 years, and the quality of most of the studies was considered to be weak; doses were not reported.(Kheirmandparizi 2021)

Hypertension

Clinical data

Strong evidence supporting an effect of garlic on blood pressure is lacking. Heterogeneity exists among trials, along with poor methodology and a lack of standardization of preparations used. Systematic reviews and meta-analyses have suggested only modest reductions in systolic blood pressure (SBP), particularly when baseline SBP was at least 140 mm Hg. One meta-analysis of 5 trials of adequate quality found garlic had no effect on blood pressure.(Simons 2009) In another review pooling data from 10 trials, SBP was decreased 16 mm Hg (95% CI, −26 to −6) only when participants had a baseline SBP of more than 140 mm Hg(Reinhart 2008); in a further meta-analysis of 11 trials, a mean decrease in SBP of 10 mm Hg was observed in hypertensive participants.(Ried 2010) A systematic review and meta-analysis of 7 randomized clinical trials (from 1988 to 2013) evaluating garlic for the treatment of hypertension showed similar effects on SBP (weighted mean difference [WMD], −6.71 mm Hg) as well as diastolic blood pressure (DBP) (WMD, −4.79 mm Hg); however, overall quality of studies could not be assessed due to insufficient reporting of methodology in some trials.(Xiong 2015) No effect was demonstrated among normotensive individuals.(Reinhart 2008, Ried 2010)

In a 2012 Cochrane review of randomized, placebo-controlled trials evaluating garlic preparations versus placebo for the treatment of hypertension, there were insufficient outcomes data to determine whether garlic was effective in reducing cardiovascular morbidity and mortality in patients diagnosed with hypertension.(Stabler 2012) Another systematic review and meta-analysis of randomized clinical trials (published through March 2014) comparing garlic preparations with placebo in hypertensive patients (9 studies; N=482) showed similar modest improvements in both SBP and DBP; the review included small study sizes with high heterogeneity.(Rohner 2015)

In a randomized controlled trial, elderly patients with systolic hypertension received daily placebo or 1, 2, or 4 capsules daily of aged garlic extract (240 mg, 480 mg, or 960 mg dosages, respectively [containing 0.6 mg, 1.2 mg, or 2.4 mg, respectively, of S-allylcystein]). Mean SBP was significantly reduced (by 11.8±5.4 mm Hg) in the 2-capsule group over 12 weeks compared with placebo (P=0.006) and reached a borderline significant reduction in the 4-capsule group at 8 weeks (by 7.4±4.1 mm Hg; P=0.07). The investigators cited poor compliance and less tolerability in the 4-capsule group as potential explanations for the superior result in the 2-capsule group.(Ried 2013)

In a meta-analysis of 18 randomized controlled trials (published between January 1946 and November 2013) investigating the effects of garlic on blood pressure, doses of garlic powder ranged from 300 to 2,400 mg/day for 2 to 24 weeks. Statistically significant improvements in SBP and DBP of 3.75 mm Hg and 3.39 mm Hg, respectively were observed compared with controls (P<0.001 each), with subgroup analysis favoring benefit in hypertensive patients. However, clinical significance of less than a 4 mm Hg reduction is questionable.(Wang 2015)

A double-blind, randomized, placebo-controlled study (the AGE at Heart Trial; published in 2016) investigated the effect of aged garlic extract supplementation on blood pressure and other important risk factors of cardiovascular disease, including central blood pressure and arterial stiffness. The study was conducted in 88 participants with uncontrolled hypertension who were administered 2 capsules daily of Kyolic aged garlic extract (1.2 g of aged garlic extract powder and 1.2 mg of S-allylcysteine per capsule) or placebo for 12 weeks. Statistically significant reductions from baseline were observed in SBP (mean difference, −5 mm Hg; P=0.016) but not in DBP. Subgroup analysis of the 50% to 60% of patients who were responders (defined as mean reduction by more than 3% in SBP [at least 5 mm Hg] or DBP [at least 3 mm Hg] over time compared to baseline) revealed statistically and clinically significant average reductions of 11.5 mm Hg in SBP and 6.3 mm Hg in DBP compared with placebo (P<0.001 for both). Trends toward improvement in central hemodynamic measures and lipid levels (ie, total cholesterol, LDL cholesterol) were noted. Aged garlic extract was well tolerated.(Ried 2016)

Considering the quality and heterogeneity of available clinical results, garlic cannot be recommended for the management of hypertension at this stage.(Reinhart 2008, Ried 2008, Ried 2010, Simons 2009)

Ischemic heart disease and heart failure

Clinical data

A clinical practice guideline from the American College of Physicians/American College of Cardiology Foundation/American Heart Association/American Association for Thoracic Surgery/Preventive Cardiovascular Nurses Association/Society of Thoracic Surgeons (2012) regarding management of stable ischemic heart disease recommend that treatment with garlic not be used with the intent of reducing cardiovascular risk or improving outcomes in patients with stable ischemic heart disease (strong recommendation; low-quality evidence). The focused update (2014) of recommendations affected by evolving data or opinion made no new or additional comment regarding the use of garlic for ischemic heart disease.(Fihn 2014, Qaseem 2012)

In a clinical study, effects of black garlic 20 g/day administered for 30 days in patients with heart failure due to coronary heart disease were assessed. Black garlic administration significantly increased quality of life scores and left ventricular function (P<0.05), as well as lower brain natriuretic peptide (BNP) precursor N-terminal concentrations (P<0.05).(Liu 2018)

Liver disease

Animal data

Garlic administration reduced increases in liver enzyme activities as well as exerted antioxidant effects in the livers of rats with type 1 diabetes.(Pocobelli 2010)

Clinical data

In a double-blind, randomized, placebo-controlled study of South Korean patients with mild hepatic dysfunction and elevated gamma-glutamyl transpeptidase (GGT) (N=80, with data from 75 patients included in the efficacy analysis), administration of fermented garlic extract (1.5 g/day) for 12 weeks provided a significant decrease in GGT compared to controls, but only in the subgroup of patients who consumed alcohol in the 6 weeks prior to the study (P=0.015). Additionally, ALT, global fatigue severity, and situation-specific fatigue were significantly improved with fermented garlic extract compared to controls (P=0.014, P=0.018, and P=0.011, respectively). The intervention was well tolerated, with no significant difference in adverse events observed between groups. The most frequent adverse effects reported with garlic extract were dyspepsia and gastritis.(Kim 2017)

Metabolic syndrome

Clinical data

In a clinical study of 40 patients with metabolic syndrome, garlic 100 mg/kg twice daily for 4 weeks significantly reduced waist circumstance (P=0.04), SBP and DBP (P<0.001), triglycerides (P=0.006), and fasting plasma glucose (P<0.0001), and increased HDL cholesterol (P<0.0001). However, no significant difference in body mass index was noted (P=0.341).(Choudhary 2018)

Osteoarthritis

Animal and in vitro data

In in vitro and animal models of osteoarthritis, allicin exerted anti-inflammatory effects through inhibition of proinflammatory mediators, as well as prevented the destruction of cartilage.(Qian 2018)

Clinical data

In a randomized, double-blind, placebo-controlled study, 80 postmenopausal overweight or obese women (50 to 75 years of age) with mild to moderate knee osteoarthritis were randomized to receive garlic 500 mg twice daily or placebo for 12 weeks. Garlic supplementation significantly reduced levels of resistin, a proinflammatory adipocytokine (P=0.008), as well as visual analog pain scale scores (P=0.043). However, tumor necrosis factor alpha levels were not significantly reduced.(Dehghani 2018)

Peripheral vascular disease

Clinical data

A 2013 Cochrane systematic review found little data regarding benefit of garlic in peripheral arterial occlusive disease. The reviewers found one 12-week study (N=78) that showed no difference between garlic and placebo for pain-free walking distance improvement, SBP and DBP, heart rate, or brachial or ankle pressures.(Jepson 2013)

Wound healing

Clinical data

In a small, unblind, randomized, controlled study, the effect of topical garlic ointment on scar formation and wound healing was investigated in 17 patients with 2 skin lesions that needed excision. After applying 30% freshly compounded garlic ointment to 1 excision and vehicle (Vaseline) to the other (twice daily for 2 weeks) to a total of 34 wounds, physician assessment observed garlic to provide superior healing in a higher proportion of wounds at the 2- and 4-week postop evaluations (65% and 88%, respectively). Patient assessments supported these results (59% and 76%, respectively), and more patients indicated that garlic provided more aesthetically appealing wounds and less discomfort than vehicle alone at both 2 and 4 weeks. Digitized photographic analysis indicated significantly less erythema with garlic at 2 weeks (P=0.02) and no difference between treatments at 4 weeks.(Alhashim 2020)

Dosing

Garlic dosage recommendations are complicated by the volatility and instability of important constituents and by the availability of multiple products, such as deodorized garlic, aged extracts (a popular deodorized form of garlic), and distilled oils. Timed-release garlic powder capsules have also been used in clinical studies.Sobenin 2008

Suggested daily doses include 2 to 5 g of fresh raw garlic; 0.4 to 1.2 g of dried garlic powder; 2 to 5 mg of garlic oil; 300 to 1,000 mg of garlic extract (as solid material); and 2,400 mg/day of aged garlic extract (liquid).Matsumoto 2016, WHO 1999

In a meta-analysis evaluating effects of garlic on blood pressure, doses of garlic powder ranged from 300 to 2,400 mg/day for 2 to 24 weeks.Wang 2015

Garlic should be administered with food to prevent GI adverse reactions.WHO 1999

Pregnancy / Lactation

Garlic may be used cautiously during pregnancy and lactation.LactMed 2018, WHO 1999

A Cochrane review identified only 1 trial (N=100 women) evaluating effects of garlic on prevention of preeclampsia and its complications; results showed no clear difference between garlic and placebo ingestion in preeclampsia and gestational hypertension risk, nor in incidences of cesarean delivery and perinatal mortality.Meher 2006

Anecdotal evidence suggests that maternal consumption of garlic may result in colic. However, evidence is conflicting. Administration of 1.5 g of garlic has been associated with increased infant attachment time to the breast and sucking time.Amer 2015, LactMed 2018

Interactions

Conflicting evidence exists regarding garlic's impact on the CYP-450 system, namely CYP1A2, 2D6, and 3A4. However, it is suggested that garlic's interactions are likely mediated through its impact on P-glycoprotein rather than the CYP-450 systems.(Asher 2017, Wanwimolruk 2014)

Agents with antiplatelet properties: Herbs (anticoagulant/antiplatelet properties) may enhance the adverse/toxic effect of agents with antiplatelet properties. Bleeding may occur. Consider therapy modification.(Leite 2016, Mousa 2010, Spolarich 2007, Stanger 2012, Ulbricht 2008, Wilson 2018)

Anticoagulants: Herbs (anticoagulant/antiplatelet properties) may enhance the adverse/toxic effect of anticoagulants. Bleeding may occur. Consider therapy modification.(Leite 2016, Mousa 2010, Spolarich 2007, Stanger 2012, Ulbricht 2008, Wilson 2018)

Antidiabetic drugs: Garlic may enhance the effects of antidiabetes drugs such as metformin. Monitor therapy.(Gupta 2017)

Herbs (anticoagulant/antiplatelet properties): Herbs (anticoagulant/antiplatelet properties) may enhance the adverse/toxic effect of other herbs (anticoagulant/antiplatelet properties). Bleeding may occur. Consider therapy modification.(Mousa 2010, Spolarich 2007, Stanger 2012, Ulbricht 2008)

Hypoglycemia-associated agents: Herbs (hypoglycemic properties) may enhance the hypoglycemic effect of hypoglycemia-associated agents. Monitor therapy.(Hui 2009)

Nonsteroidal anti-inflammatory agents: Herbs (anticoagulant/antiplatelet properties) may enhance the adverse/toxic effect of nonsteroidal anti-inflammatory agents. Bleeding may occur. Consider therapy modification. (Mousa 2010, Spolarich 2007, Stanger 2012, Ulbricht 2008)

Protease inhibitors: Garlic may decrease the serum concentration of protease inhibitors. Avoid combination.(Cloarec 2017, Duncan 2013, Gallicano 2003, Hajda 2010, Invirase September 2020, Piscitelli 2002)

Salicylates: Herbs (anticoagulant/antiplatelet properties) may enhance the adverse/toxic effect of salicylates. Bleeding may occur. Consider therapy modification.(Mousa 2010, Spolarich 2007, Stanger 2012, Ulbricht 2008)

Thrombolytic agents: Herbs (anticoagulant/antiplatelet properties) may enhance the adverse/toxic effect of thrombolytic agents. Bleeding may occur. Consider therapy modification.(Mousa 2010, Spolarich 2007, Stanger 2012, Ulbricht 2008)

Adverse Reactions

Although garlic is used extensively for culinary purposes with few adverse effects, the safety of long-term use of concentrated extracts is unclear. Randomized clinical trials and observational studies of garlic-coated tablets have reported mild GI adverse reactions, including nausea, bloating, and flatulence. Obstruction of the small intestine, epigastric and esophageal pain, dysphagia, hematemesis, and hematochezia have been reported in single case reports.Borelli 2007, Ergül 2012,

Controlled clinical trials and observational studies have consistently documented that body odor and malodorous breath are the most common complaints after ingestion of garlic preparations.Borelli 2007 Consumption of milk prior to garlic ingestion may reduce bad breath.Hansanugrum 2010

Ingestion of garlic may rarely cause anaphylaxis.Rivlin 2009, Treudler 2015 Asthma and immunoglobulin E–mediated hypersensitivity, including dermatitis, have been documented. Concurrent drug therapy with omeprazole may facilitate sensitization to food allergens.Treudler 2015 Cross-sensitivity to other members of the Liliaceae family, Amaryllis family (Amaryllidaceae), and asparagus may occur.Añibarro 1997, Borelli 2007, Eming 1999, Sanchez-Hernandez 2000, Takeuchi 2011, Treudler 2015 A case of pneumonia induced by black garlic was documented in a 77-year-old female who presented with dyspnea and coughing. Drug-induced lymphocyte stimulation test results for both black and raw garlic were positive. Symptoms improved over the next 9 months with a tapered prednisolone regimen and cessation of garlic.Suzuki 2016

Case reports of garlic burns in children and adults following topical application of crushed or cut garlic exist,Schimmel 2019, Sharp 2018, Strickler 2019 including an unusually severe case that developed when a garlic and salt paste was applied and covered for 3 hours with an occlusive dressing.Xu 2014 Garlic can cause a direct chemical burn as well as cause a type IV sensitivity reaction in sensitized people. This type of reaction can be caused by garlic's sulfur-containing compounds, with the main agent being diallyl disulfide. The risk for burns is increased when the site of topical garlic application is covered with occlusive dressings.Sharp 2018, Strickler 2019

Case reports have documented alterations in coagulation, spinal epidural hematoma, increased clotting time, postoperative bleeding, and retrobulbar hemorrhage after excessive dietary intake of garlic or garlic tablets.Beckert 2007, Borelli 2007, Gravas 2010, Mousa 2010, Scharbert 2007 Hypotensive effects, Ménière disease, and myocardial infarction have also been documented.Borelli 2007

Garlic should be used with caution in individuals at risk of hypotensionReinhart 2008, Ried 2010 and hypoglycemia.Ackermann 2001, Ernst 1996, Sobenin 2008

In the 2016 Scientific Statement by the American Heart Association regarding drugs that may cause or exacerbate heart failure, garlic was recognized as a product with antiplatelet activity that may increase bleeding risk when used with anticoagulants. The guidance noted that nutraceuticals are not recommended for the management of heart failure symptoms or for the secondary prevention of cardiovascular events, and that nutritional supplements are not recommended for the treatment of heart failure.Page 2016

Toxicology

Information regarding the toxicology of garlic is limited. Garlic (10%, 15%, and 30% crude garlic) fed to rats for 30 days resulted in decreased testosterone secretion and altered spermatogenesis.Hammami 2008 Garlic can be toxic to dogs and cats, increasing the risk for the development of anemia. A case report documents severe anemia and hypertension from toxic erythrocyte hemolysis in a dog that ingested baked garlic.Kang 2010

References

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