Garlic

What it is

Garlic is the bulb of Allium sativum, eaten as a food and sold as powdered tablets, aged extracts and oils. Supplements are usually described by their allicin yield or their S-allylcysteine content, and the single trial Cochrane included for the common cold used a garlic supplement with 180 mg of allicin content. Trials have compared raw garlic with commercial preparations directly: one gave the equivalent of a 4 g clove a day, six days a week, for six months. Garlic's characteristic smell comes from volatile organic sulfur compounds, which the burn-injury literature identifies as the same compounds responsible for skin damage after raw garlic is applied to skin.

What the research says

Garlic's best-supported effect is a modest reduction in blood pressure in people who already have hypertension: meta-analyses report systolic reductions of roughly 4 to 7 mmHg, though the reviews themselves call for more rigorous trials. The cholesterol claim did not survive testing; a six-month randomised trial of raw garlic and two commercial supplements found no effect on LDL. Cochrane found only one eligible trial for the common cold and called the evidence insufficient. In a 22-year follow-up of a large randomised trial, garlic supplementation did not significantly reduce gastric cancer incidence, though mortality was borderline lower. Documented harms include odour, gastrointestinal upset, chemical burns from raw garlic on skin, increased bleeding risk, and a large reduction in blood levels of the HIV drug saquinavir.

Evidence grade: Limited evidence.

What goes wrong

The same 2025 review reported more side effects in the garlic groups than in the control groups, but its own two statements about whether that difference was significant do not agree. (Source 1)

• Meta-analysis, Certainty not rated.
• Size: 10 randomised controlled trials in people with hypertension.
• Who: patients with a clear diagnosis of hypertension.
• How long: intervention durations varied between trials.
• Result: Adverse side effects RR = 1.79 (95% CI 1.18 to 2.72); most commonly gastrointestinal discomfort and bad breath. The paper's abstract describes this same difference as not statistically significant.
• Funding: not stated in the abstract.

Limit of this finding: This paper says two different things about side effects, and a reader needs both. Its abstract states that the rate of adverse reactions in the garlic group was slightly higher but not statistically significant. The safety section quoted here gives the number behind that same comparison as a risk ratio of 1.79, with a 95% confidence interval running from 1.18 to 2.72 - a range that does not include 1, which by the usual statistical test means the increase would be counted as real rather than a chance finding. The two statements cannot both be right and the paper does not explain the difference. What a reader can take from it: people given garlic in these trials reported more side effects than the comparison groups, and those side effects were mild, mostly digestive discomfort and bad breath. What a reader should not take from it: a settled figure for how much side effects increase, or a conclusion that garlic has been shown to cause significantly more harm, because the paper's own abstract calls this very difference non-significant.

Findings from the meta-analysis reveal that the occurrence of negative side effects in the group receiving garlic extract was marginally greater compared to the control group (RR = 1.79, 95% CI: 1.18 to 2.72).

Garlic caplets cut blood levels of the HIV protease inhibitor saquinavir by about half in healthy volunteers. (Source 2)

• Blood level study, Certainty not rated.
• Size: 10 healthy volunteers.
• Who: healthy adults given saquinavir 1200 mg three times daily with meals, then garlic caplets twice daily.
• How long: 41 doses of garlic caplets over study days 5-25, with a 10-day washout.
• Result: Mean saquinavir AUC fell 51%, trough levels fell 49% and mean Cmax fell 54%; after washout values returned only to 60%-70% of baseline.
• Funding: not stated in the abstract.

In the presence of garlic, the mean saquinavir area under the curve (AUC) during the 8-h dosing interval decreased by 51%, trough levels at 8 h after dosing decreased by 49%, and the mean maximum concentrations (Cmax) decreased by 54%.

A systematic review of case reports documented 39 patients burned by applying raw garlic to skin, most with second-degree burns. (Source 3)

• Systematic review, Certainty not rated.
• Size: 39 patients across 32 articles.
• Who: people who applied raw garlic to skin or mucosa, often as a home remedy.
• How long: hours of contact, frequently under occlusion.
• Result: Most cases were second-degree burns; some circumstances produced necrotic tissue; legs were the most commonly affected body part.
• Funding: not stated in the abstract.

In most of the cases, garlic caused second-degree burns, although some circumstances can cause formation of necrotic tissue.

NCCIH's position is that garlic supplements may increase bleeding risk and interact with anticoagulants and aspirin. (Source 4)

• Official position, Certainty not rated.
• Size: not applicable.
• Who: general public, particularly people facing surgery or taking blood thinners.
• How long: page last updated February 2025.
• Result: No numeric estimate given; described as an increased risk of bleeding.
• Funding: US government agency (National Center for Complementary and Integrative Health)

Taking garlic supplements may increase the risk of bleeding. If you take garlic supplements, make sure to tell your health care providers.

What the evidence supports

A meta-analysis of seven placebo-controlled trials in people with hypertension found garlic lowered systolic and diastolic blood pressure. (Source 5)

• Meta-analysis, Certainty not rated.
• Size: seven randomized, placebo-controlled trials.
• Who: patients with diagnosed hypertension.
• How long: varied by trial; the review calls for long-term follow-up studies.
• Result: Systolic BP weighted mean difference -6.71 mmHg (95% CI -12.44 to -0.99; P = 0.02); diastolic BP -4.79 mmHg (95% CI -6.60 to -2.99; P < 0.00001)
• Funding: not stated in the abstract.

Compared with the placebo, this meta-analysis revealed a significant lowering effect of garlic on both systolic BP (WMD: -6.71 mmHg; 95% CI: -12.44 to -0.99; P = 0.02) and diastolic BP (WMD: -4.79 mmHg; 95% CI: -6.60 to -2.99; P < 0.00001).

A 2025 meta-analysis of ten randomised trials reported reductions in both systolic and diastolic blood pressure with garlic. (Source 6)

• Meta-analysis, Certainty not rated.
• Size: 10 randomised controlled trials, selected from 1,877 articles retrieved.
• Who: patients with a clear diagnosis of hypertension.
• How long: intervention durations varied between trials; subgroup effects were seen at 8 weeks.
• Result: Systolic BP effect size -4.21 (95% CI -5.74 to -2.69, P < 0.001); diastolic BP effect size -3.13 (95% CI -4.42 to -1.84, P < 0.001)
• Funding: not stated in the abstract.

The results indicated a significant reduction in systolic blood pressure (SBP) (effect size: −4.21, 95% CI: −5.74 to −2.69, P < 0.001) and diastolic blood pressure (DBP) (effect size: −3.13, 95% CI: −4.42 to −1.84, P < 0.001) in the garlic intervention group compared to the control group.

In the same trial, gastric cancer mortality was lower in the garlic group, but the confidence interval reached 1.00, so the result does not rule out no effect. (Source 7)

• Randomized trial, Low certainty.
• Size: 3,365 participants; 94 gastric cancer deaths.
• Who: Chinese adults in a high gastric cancer risk region.
• How long: 7.3 years of supplementation, 22 years of follow-up.
• Result: Fully adjusted hazard ratio for garlic supplementation 0.66 (95% CI 0.43 to 1.00)
• Funding: not stated in the abstract.

Limit of this finding: The paper's own summary sentence says all three interventions showed significant reductions in gastric cancer mortality, but the garlic hazard ratio it gives, 0.66 with a 95% confidence interval of 0.43 to 1.00, reaches exactly 1.00, which means no reduction cannot be ruled out. The same trial found no reduction in gastric cancer incidence with garlic (0.81, 0.57 to 1.13). A reader should treat the garlic mortality result as borderline and unconfirmed, not as evidence that garlic prevents death from stomach cancer.

All three interventions showed significant reductions in gastric cancer mortality: fully adjusted hazard ratio for H pylori treatment was 0.62 (95% confidence interval 0.39 to 0.99), for vitamin supplementation was 0.48 (0.31 to 0.75), and for garlic supplementation was 0.66 (0.43 to 1.00).

What the evidence does not support

A six-month randomised trial in adults with moderate hypercholesterolemia found that neither raw garlic nor two commercial supplements changed LDL cholesterol. (Source 8)

• Randomized trial, Certainty not rated.
• Size: 192 adults.
• Who: adults with LDL cholesterol concentrations of 130 to 190 mg/dL.
• How long: 6 days a week for 6 months.
• Result: No statistically significant effects on LDL cholesterol for raw garlic, powdered supplement or aged extract compared with placebo.
• Funding: not stated in the abstract we retrieved; the trial was conducted at Stanford.

None of the forms of garlic used in this study, including raw garlic, when given at an approximate dose of a 4-g clove per day, 6 d/wk for 6 months, had statistically or clinically significant effects on LDL-C or other plasma lipid concentrations in adults with moderate hypercholesterolemia.

Cochrane found only one eligible randomised trial of garlic for the common cold and judged the evidence insufficient. (Source 9)

• Systematic review, Certainty not rated.
• Size: one included trial of 146 participants, from eight potentially relevant trials identified.
• Who: adults taking a garlic supplement containing 180 mg of allicin, or placebo, once daily.
• How long: 12 weeks.
• Result: The single included trial gave a garlic supplement with 180 mg of allicin content once daily for 12 weeks: 24 colds in the garlic group versus 65 on placebo (P value < 0.001) and fewer days of illness (111 versus 366), but days to recovery were similar (4.63 versus 5.63) and the trial relied on self-reported episodes.
• Funding: not stated in the abstract.

There is insufficient clinical trial evidence regarding the effects of garlic in preventing or treating the common cold.

Cochrane's verdict on the wider garlic-and-colds literature was that it rests largely on poor-quality evidence. (Source 9)

• Systematic review, Certainty not rated.
• Size: one trial met inclusion criteria out of eight screened.
• Who: people using garlic to prevent or treat colds.
• How long: not applicable.
• Result: No pooled estimate was possible; the single trial reported rash and odour as adverse effects.
• Funding: not stated in the abstract.

Claims of effectiveness appear to rely largely on poor-quality evidence.

In a 22-year follow-up of a randomised trial in a high-risk Chinese population, garlic supplementation did not significantly reduce gastric cancer incidence. (Source 7)

• Randomized trial, Certainty not rated.
• Size: 3,365 residents randomised; 151 incident gastric cancers and 94 gastric cancer deaths over 1995-2017.
• Who: residents of Linqu County, Shandong province, China, a high-risk region for gastric cancer.
• How long: garlic extract and oil for 7.3 years (1995-2003), with follow-up through 2017.
• Result: Gastric cancer incidence odds ratio for garlic supplementation 0.81 (95% CI 0.57 to 1.13), not significant, against 0.64 (0.46 to 0.91) for vitamin supplementation.
• Funding: not stated in the abstract; a long-term publicly reported intervention trial.

Limit of this finding: The paper's own summary sentence says all three interventions showed significant reductions in gastric cancer mortality, but the garlic hazard ratio it gives, 0.66 with a 95% confidence interval of 0.43 to 1.00, reaches exactly 1.00, which means no reduction cannot be ruled out. The same trial found no reduction in gastric cancer incidence with garlic (0.81, 0.57 to 1.13). A reader should treat the garlic mortality result as borderline and unconfirmed, not as evidence that garlic prevents death from stomach cancer.

Incidence decreased significantly with vitamin supplementation but not with garlic supplementation (0.64, 0.46 to 0.91 and 0.81, 0.57 to 1.13, respectively).

NCCIH also states that garlic does not seem to reduce stomach cancer risk and that immune-support claims rest on very little research. (Source 4)

• Official position, Certainty not rated.
• Size: not applicable.
• Who: general public.
• How long: page last updated February 2025.
• Result: No numeric estimate; a 2022 review is cited as identifying only two small, weak studies suggesting possible immune benefit.
• Funding: US government agency (NCCIH)

Consuming garlic does not seem to reduce the risk of stomach cancer. It's uncertain whether garlic influences the risk of colorectal cancer.

Where the evidence is mixed

The same review cautioned that garlic should not yet be recommended for hypertension because the trials were not rigorous enough. (Source 5)

• Meta-analysis, Certainty not rated.
• Size: seven trials.
• Who: patients with hypertension.
• How long: short-term trials without primary clinical endpoints.
• Result: No serious adverse events were reported in any of the trials, but the authors call for trials with primary endpoints and long-term follow-up.
• Funding: not stated in the abstract.

However, more rigorously designed randomized controlled trials focusing on primary endpoints with long-term follow-up are still warranted before garlic can be recommended to treat hypertensive patients.

The same 2025 review concluded the overall impact on hypertension may be limited and that adverse effects should be weighed. (Source 6)

• Meta-analysis, Certainty not rated.
• Size: 10 randomised controlled trials.
• Who: patients with hypertension.
• How long: varied.
• Result: Benefit concentrated in subgroups (obese, aged 50-60, higher baseline diastolic pressure); gastrointestinal discomfort and bad breath were the most common side effects.
• Funding: not stated in the abstract.

Limit of this finding: The paper contradicts itself on adverse events. Its abstract says the higher rate in the garlic groups was 'not statistically significant', but its safety section reports RR = 1.79 with a 95% confidence interval of 1.18 to 2.72, which does not include 1 and therefore is statistically significant by the usual test. A reader should conclude that garlic groups reported more side effects than control groups, mostly digestive upset and bad breath, and should not rely on the abstract's claim that the difference was not significant.

However, the overall impact on hypertension may be limited. Potential gastrointestinal and other adverse effects should be considered in clinical practice.

Where the research disagrees

How strong the blood pressure evidence for garlic is

• Wang and colleagues, Phytomedicine meta-analysis, 2015, meta-analysis of seven randomised placebo-controlled trials in hypertensive patients: The present review suggests that garlic is an effective and safe approach for hypertension. (Source 5)
• NCCIH, page updated February 2025, government agency summary of the trial literature: Limited evidence suggests that garlic supplements may reduce blood pressure to a small extent in people who have high blood pressure. (Source 4)

Whether garlic affects gastric cancer

• Li and colleagues, BMJ, 2019, 22-year follow-up of a blinded randomised placebo-controlled factorial trial in 3,365 people: All three interventions showed significant reductions in gastric cancer mortality: fully adjusted hazard ratio for H pylori treatment was 0.62 (95% confidence interval 0.39 to 0.99), for vitamin supplementation was 0.48 (0.31 to 0.75), and for garlic supplementation was 0.66 (0.43 to 1.00). (Source 7)
• NCCIH, page updated February 2025, government agency summary of the evidence: Consuming garlic does not seem to reduce the risk of stomach cancer. (Source 4)

How much

• Reference intake: No dietary reference intake or recommended daily amount has been set for garlic by any body we could reach; it is a food and a herbal supplement, not an essential nutrient. NCCIH frames the question as duration of safe use in research rather than a recommended amount (page updated February 2025). (Source 4)
• Upper limit: No tolerable upper intake level or acceptable daily intake has been set. The practical limits described by NCCIH are bleeding risk with supplements, severe skin irritation and chemical burns from fresh raw garlic used topically, and caution in pregnancy above food amounts (page updated February 2025). (Source 4)
• Studied: A six-month randomised trial gave the equivalent of an approximate 4 g clove of raw garlic per day, six days a week, alongside a powdered supplement and an aged extract. (Source 8)
• Studied: The single trial Cochrane included for the common cold gave a garlic supplement with 180 mg of allicin content, once daily for 12 weeks. (Source 9)
• Studied: The Linqu County trial gave garlic extract and oil for 7.3 years (1995-2003). (Source 7)

A common belief, and what the research shows

The belief: Garlic lowers cholesterol.

What the research shows: The best test of this failed, in the population where it was most likely to show something. A six-month randomised trial of 192 adults with raised LDL compared raw garlic against two commercial supplements and placebo, and found "no statistically significant effects of the 3 forms of garlic on LDL-C concentrations". The authors concluded that "None of the forms of garlic used in this study, including raw garlic, when given at an approximate dose of a 4-g clove per day, 6 d/wk for 6 months, had statistically or clinically significant effects on LDL-C or other plasma lipid concentrations in adults with moderate hypercholesterolemia." NCCIH still describes a small effect in people with high cholesterol, which is the more optimistic reading of a mixed literature.

Questions and answers

What is it?

Garlic is the bulb of Allium sativum, used as a food and sold as powdered tablets, aged extracts and oils. Supplements are typically described by allicin yield. Its smell and much of its biological activity come from volatile organic sulfur compounds, which are also what burns skin when raw garlic is applied to it. (Source 3)

What does it do in the body?

In people with high blood pressure, garlic preparations produce a modest reduction in blood pressure across randomised trials. Claims about cholesterol, colds, immunity and stomach cancer are much weaker or have failed direct testing. Garlic is a food, not a nutrient with a defined function in the body. (Source 5)

Is it good or bad for you?

As a food, garlic has been consumed safely and has been used in research for as long as seven years. As a concentrated supplement it carries a bleeding risk and interacts with medicines, and a 2025 meta-analysis found more adverse reactions in garlic groups. Applied raw to skin it causes chemical burns. Context decides: the same plant is benign in cooking and harmful under a bandage. (Source 4)

How do you get more of it?

Garlic is a food, so intake comes from cooking with it or from supplements sold as powders, aged extracts or oils. The single trial Cochrane included for the common cold gave a garlic supplement with 180 mg of allicin content, once daily for 12 weeks. A six-month cholesterol trial gave the equivalent of about a 4 g clove of raw garlic a day, six days a week. This describes what trials gave people, not a recommendation. (Source 9)

If it is harmful, what reduces it?

Garlic is not stored in the body and clears with ordinary intake. Where it has caused harm, treatment is of the injury rather than of the garlic: in the reported burn cases, care was mainly symptomatic. NCCIH advises telling health care providers about garlic supplements, particularly before surgery or alongside anticoagulants or aspirin. (Source 3)

Why might someone be low in it or missing it?

This does not apply in the usual sense: garlic is not an essential nutrient, so nobody is deficient in it. What the trials show is the reverse of a deficiency story. In adults with moderate hypercholesterolemia, six months of raw garlic or commercial garlic supplements left LDL cholesterol unchanged, so having little garlic in the diet has no known cholesterol consequence. (Source 8)

Which whole foods contain it or feed it?

The whole food is the garlic bulb itself, eaten raw or cooked. Trials have compared raw garlic directly against commercial preparations, and the raw form performed no better for cholesterol. Concentrated extracts and oils are processed forms of the same bulb. (Source 8)

What happens if you do not have it?

Nothing is known to go wrong from not eating garlic. In the placebo groups of the trials, cholesterol did not worsen and cancer risk was measured against people who took no garlic at all for over seven years. The realistic question is whether taking it changes outcomes, and for most outcomes the answer has been no or uncertain. (Source 7)

How can you test for it?

No validated clinical test of a person's garlic status exists in the literature we searched, because there is no deficiency state to measure. We searched for garlic biomarker and status testing alongside the trial and review literature and the NCCIH monograph, and found only measurements of garlic's effects, such as blood pressure and plasma lipids, rather than of garlic itself. (Source 4)

We searched: WebSearch for garlic status and biomarker testing, and the safety and effectiveness sections of the NCCIH garlic monograph, the Cochrane common cold review, the Phytomedicine and Frontiers in Nutrition meta-analyses and the Stanford lipid trial; none describes a test of garlic status in a person.

References

1. Frontiers in Nutrition. Meta-analysis on the safety and efficacy of long-term garlic consumption as an adjunctive treatment for hypertension. 2025. DOI 10.3389/fnut.2025.1656809. Read the source
2. Clinical Infectious Diseases. The Effect of Garlic Supplements on the Pharmacokinetics of Saquinavir. 2002. PMID 11740713, DOI 10.1086/324351. Read the source
3. The American Journal of Emergency Medicine. Garlic burn injuries- a systematic review of reported cases. 2021. PMID 33571752, DOI 10.1016/j.ajem.2021.01.039. Read the source
4. National Center for Complementary and Integrative Health (NIH). Garlic: Usefulness and Safety. 2025. Read the source
5. Phytomedicine. Garlic for hypertension: A systematic review and meta-analysis of randomized controlled trials. 2015. PMID 25837272, DOI 10.1016/j.phymed.2014.12.013. Read the source
6. Frontiers in Nutrition. Meta-analysis on the safety and efficacy of long-term garlic consumption as an adjunctive treatment for hypertension. 2025. DOI 10.3389/fnut.2025.1656809. Read the source
7. The BMJ. Effects of Helicobacter pylori treatment and vitamin and garlic supplementation on gastric cancer incidence and mortality: follow-up of a randomized intervention trial. 2019. PMID 31511230, DOI 10.1136/bmj.l5016. Read the source
8. Archives of Internal Medicine. Effect of raw garlic vs commercial garlic supplements on plasma lipid concentrations in adults with moderate hypercholesterolemia: a randomized clinical trial. 2007. PMID 17325296, DOI 10.1001/archinte.167.4.346. Read the source
9. Cochrane Database of Systematic Reviews. Garlic for the common cold. 2014. PMID 25386977, DOI 10.1002/14651858.CD006206.pub4. Read the source