Garlic pills nudge blood pressure down, but a Stanford trial found no cholesterol benefit

Key takeaways

• In a 2020 pooled analysis of 12 trials with 553 people with high blood pressure, garlic products lowered the top number by about 8 points; the author led the four aged-extract trials in it and had travel support from their maker, Wakunaga⁵.
• A 2007 Stanford trial gave 192 adults raw garlic, aged garlic extract or garlic powder for six months, and none lowered LDL cholesterol²³.
• The Cochrane review of garlic for colds found just one eligible trial, with 146 people, and called the evidence insufficient⁷.
• A European Medicines Agency draft monograph advises avoiding garlic for 7 days before surgery because of bleeding risk and rules out garlic products with the HIV drugs saquinavir and ritonavir¹².
• Fresh raw garlic held on the skin can cause chemical burns, according to the National Center for Complementary and Integrative Health¹.

Over six months, a Stanford University team assembled more than 30,000 sandwiches².

Some volunteers got raw garlic stirred into the sandwich spread. Others got plain spread plus tablets of one of two top-selling garlic supplements, or a dummy pill². All 192 had moderately high LDL cholesterol, the “bad” kind, averaging about 140 mg/dL².

Every garlic group got the same daily dose, matched to one average 4-gram clove, six days a week². Lab analyses confirmed that the products stayed stable and delivered comparable amounts of garlic².

The results, published in the Archives of Internal Medicine in 2007, were flat. LDL barely moved in any group²³. Garlic “just didn’t work,” said senior author Christopher Gardner².

Garlic’s record looks like that across the board: a superb food, a few small real effects, and a reputation that runs ahead of the trials. We walk through what the evidence supports, what it doesn’t, and how to use garlic well.

One bulb, several very different products

Garlic is the bulb of Allium sativum¹². Ancient Egyptian, Greek and Indian writings describe it as a remedy for headaches, pneumonia, throat ailments and digestive troubles¹.

An unbroken clove barely smells. Its sulfur compound, alliin, and the enzyme that acts on it, alliinase, are stored in separate cells¹⁰. Crush or chop the clove and they meet, forming allicin, the source of garlic’s sharp smell¹⁰.

Allicin is fragile. It quickly breaks down into other sulfur compounds, such as diallyl disulfide¹⁰. Heat changes things again. Cooking a whole clove switches off alliinase, so little allicin forms, while frying crushed garlic in oil creates a different compound called ajoene¹⁰.

That chemistry is why garlic products differ so much:

• Garlic powder tablets are dried garlic. Some are coated so they dissolve in the intestine rather than the stomach¹⁰.
• Aged garlic extract starts as garlic left to steep in a water-alcohol mix for an extended period. The result is low in smell and high in S-allyl cysteine, a sulfur compound that dissolves in water¹⁰.
• Garlic oil is made by steam-distilling crushed garlic, which collects its oily sulfur compounds¹⁰.

Europe treats garlic as a traditional medicine.

In a 2016 draft monograph, the European Medicines Agency’s herbal committee listed two uses “exclusively based upon long-standing use”¹². One was helping to prevent hardening of the arteries, as an add-on. The other was relieving cold symptoms¹².

The committee’s final assessment followed in 2017³. That status rests on long use, not on proof from trials.

Blood pressure: a small drop, with a supplement maker close to the research

The best case for garlic pills is blood pressure. In 2008, Karin Ried and colleagues pooled 11 trials, but only four reported results for people with high blood pressure⁴. In those, garlic lowered the top (systolic) number by about 8.4 points, and in three it lowered the bottom (diastolic) number by about 7.3⁴. People who started higher saw bigger drops⁴.

Ried updated the work in 2020. Across 12 trials with 553 people with high blood pressure, garlic cut systolic pressure by about 8.3 points⁵. In the eight trials that reported it, diastolic pressure fell about 5.5⁵. The trials ran a median of 12 weeks⁵.

The form and dose matter. Most trials used garlic powder at 600 to 900 milligrams a day, with doses as high as 2,400, or Kyolic aged garlic extract, usually at 1,200 milligrams a day⁵. In one Kyolic dose trial, one capsule a day wasn’t enough, and four did no better than two⁵.

Now the funding. The 2020 review notes that its author presented the work at a 2019 garlic symposium with travel sponsorship from Wakunaga, which makes Kyolic⁵. Ried also led all four Kyolic trials in the analysis⁵. The paper says the sponsor had no role in the analysis, and its competing-interests statement lists none⁵.

One of them, the 2016 AGE at Heart trial, was funded by Wakunaga of America. It enrolled 88 people with uncontrolled high blood pressure⁶. After 12 weeks on 1.2 grams of aged garlic extract a day, average pressure fell 5.0 points more than with placebo⁶. Changes in artery stiffness were not statistically significant⁶.

The 2020 review argued that garlic lowers pressure about as much as standard blood pressure drugs⁵. No trial in it compared the two directly, though. The National Center for Complementary and Integrative Health (NCCIH) puts it more cautiously: limited evidence suggests a small drop in people with high blood pressure¹.

If you have high blood pressure, keep taking prescribed medicine. Ask your clinician before adding garlic, and track your readings at home.

Cholesterol: the trial built to settle it found nothing

Gardner’s trial ran on National Institutes of Health money. That mattered, because supplement makers had paid for many of the earlier studies reporting cholesterol benefits².

The design was hard to argue with. Volunteers had LDL between 130 and 190 mg/dL³. Outside garlic experts Larry Lawson and Eric Block helped confirm the products’ quality². Monthly blood tests tracked everyone for six months².

After six months, average LDL changes were tiny: up 0.4 mg/dL with raw garlic, up 3.2 with powder, up 0.2 with aged extract and down 3.9 with placebo³. None of the changes was statistically significant³. Results were the same for people with the highest and lowest starting LDL².

When Europe’s herbal assessors reviewed the research in 2017, they called Stanford’s study well designed in its blinding, randomization, compliance and statistical power³. They also noted that newer reviews pointed to a more modest effect of garlic on blood fats, or none³.

NCCIH says garlic supplements may lower total and LDL cholesterol slightly in people with high cholesterol, the area most heavily studied¹. The two findings can coexist: a small average effect across many studies, and none in one carefully controlled trial at a clove a day. Either way, garlic is no substitute for proven cholesterol care.

Colds: the reputation rests on one small trial

In 2014, Cochrane reviewers looked for good trials of garlic for colds. They found eight studies, and only one qualified⁷.

That trial gave 146 people a daily capsule for 12 weeks: either garlic containing allicin or a dummy pill⁷. Over those three months, the garlic takers logged 24 colds and the placebo takers 65⁷. Yet a cold, once caught, lasted roughly as long in both groups⁷.

The reviewers saw weak spots. People tracked their own colds. Four garlic takers, but only one placebo taker, noticed a garlic smell when they burped, a clue that could have told them which pill they had⁷. The reviewers also warned that studies finding no benefit may never have been published⁷.

Their bottom line: the trial evidence is too thin to say whether garlic prevents or treats colds⁷. NCCIH adds that a 2022 review found only two small studies suggesting benefit for colds and flu, both with weaknesses¹.

Cancer: lab promise, little human proof

Much of garlic’s anticancer case comes from mechanism studies of its sulfur compounds, and a 2025 review says how they act in people remains incompletely understood¹⁰. To judge the human evidence, Ji Yeon Kim of the Korea Food and Drug Administration and Oran Kwon of Ewha Womans University used the U.S. FDA’s evidence-review system for health claims⁸⁹.

They found 19 human studies. There was no credible evidence that garlic lowers the risk of stomach, breast, lung or endometrial cancer⁸. Evidence for colon, prostate, esophageal, larynx, mouth, ovarian and kidney cancers was very limited⁸. As the trade publication NutraIngredients reported, the authors concluded that the evidence couldn’t back any health claim linking garlic to lower cancer risk⁹.

NCCIH agrees that eating garlic doesn’t seem to lower stomach cancer risk and calls its effect on colorectal cancer uncertain¹.

What garlic can and can’t do, rated

Use	Form and dose studied	What the research shows	Evidence level
Lowering high blood pressure	Aged garlic extract 1.2 g a day or garlic powder 600 to 2,400 mg a day, 8 to 24 weeks	About 8 points lower systolic in 12 trials; extract maker funded key trials56	Promising but early
Lowering LDL cholesterol	Raw garlic, powder or aged extract, about one 4-gram clove a day, 6 months	Mixed: no change in the Stanford trial; NCCIH says supplements may lower LDL slightly123	Promising but early
Lowering blood sugar in diabetes	Supplements	May lower it slightly, per NCCIH1	Promising but early
Preventing colds	Allicin capsule daily for 12 weeks	One trial of 146 people; fewer self-reported colds7	Not enough evidence
Shortening colds	Same trial	Recovery time about the same7	Not enough evidence
Preventing stomach cancer	Garlic in the diet, observational studies	No credible evidence of lower risk18	Evidence says no, or unsafe
Preventing other cancers	Garlic in the diet, observational studies	Very limited or uncertain18	Not enough evidence
Raw garlic on skin as a remedy	Fresh crushed garlic held on skin	Can cause severe irritation and chemical burns1	Evidence says no, or unsafe

The safety list: bleeding, surgery, HIV drugs and burns

Garlic is part of cuisines worldwide, and NCCIH notes that oral garlic has been used safely in studies lasting up to seven years¹. Supplements and remedies bring specific cautions.

Bleeding. Garlic supplements may raise bleeding risk, NCCIH says, so tell your providers if you take blood thinners or aspirin¹. The European Medicines Agency’s committee advises caution with anticoagulant and antiplatelet drugs because garlic may lengthen bleeding time¹². Our article on habits that clash with medicines covers warfarin.

Surgery. The European committee advises avoiding garlic products for 7 days before surgery because of the risk of bleeding afterward¹². Tell your surgeon about any supplement.

HIV medicine. In a study by Piscitelli and colleagues, presented in 2001 and published in Clinical Infectious Diseases in 2002, 10 volunteers without HIV took garlic capsules twice a day for about three weeks. Afterward, their blood levels of the HIV drug saquinavir came out about half as high as before garlic¹³¹⁴. Three weeks after stopping garlic, levels still hadn’t returned to baseline¹³.

The researchers suspected garlic speeds up drug-processing enzymes in the gut lining. They advised people relying on saquinavir as their only protease inhibitor, the drug’s class, to avoid garlic supplements¹⁴. They couldn’t say whether food garlic or other products would do the same¹⁴. Europe’s draft monograph lists saquinavir with ritonavir as a reason not to use garlic products¹².

Skin. Fresh raw garlic held on the skin can cause severe irritation and chemical burns¹. Allergic skin reactions can also be severe¹². Don’t tape garlic to skin, and see a clinician for any blistering burn.

Pregnancy and children. NCCIH says oral garlic above food amounts may not be safe during pregnancy or breastfeeding¹. The European committee doesn’t recommend its garlic products in pregnancy or breastfeeding, or for anyone under 12 for colds, or under 18 for artery health¹².

Everyday side effects. These include bad breath, body odor, belly pain, gas and nausea¹. Headache, dizziness and heavy sweating have also been reported¹².

Garlic in the kitchen: crush it, let it sit, then cook

Food is still the most dependable way to use garlic. A few habits help.

Crush first. Since allicin forms only when cells break, crushing or chopping comes before heat¹⁰. A 2025 review cites a study in which garlic crushed lightly before cooking kept more of its anti-clotting activity¹⁰.

Then wait. A 2001 laboratory study in the Journal of Nutrition found that a minute in the microwave wiped out garlic’s anticancer activity. Letting crushed garlic sit for 10 minutes before heating kept some of it¹¹. That finding comes from lab research, not human trials, but the habit costs you nothing.

Expect less from cooked garlic. Researchers can estimate how much allicin reaches the body by measuring a sulfur compound on the breath. By that yardstick, garlic minced with an acid reached 66%, while roasted garlic managed about 30%, pickled 19% and boiled just 16%, according to studies gathered in the 2025 review¹⁰. Enteric-coated tablets ranged from 36% to 104% on the same scale and dropped with a high-protein meal¹⁰. Wakunaga paid that review’s publication costs¹⁰.

About your breath. The sulfur compounds you smell on your breath are the same ones researchers measure¹⁰. Aged extract, built on water-soluble compounds, has little odor¹⁰.

About your gut. The 2025 review cites a trial in which high doses of raw garlic on an empty stomach caused gas and stomach upset¹⁰. Garlic is also rich in fructans, which can bloat people with irritable bowel syndrome; our article on prebiotic foods explains.

About garlic in oil. Homemade garlic oil needs special handling to avoid botulism. Our article on respecting nature’s risks covers the rules.

Natural health has always treated food as medicine’s first line. Garlic fits that idea well. It makes vegetables, beans and whole grains taste wonderful, so you reach for them more often. That may be one of its most useful health benefits, even though it’s hard to measure.

Garlic profile card

	Kitchen garlic	Garlic pill
Plant and part	Allium sativum bulb12	Same, as powder, coated tablet, aged extract or oil10
Amount studied	About one 4-gram clove a day2	Aged extract 1.2 g a day; powder 600 to 2,400 mg a day5
Best-supported use	Flavor for healthy meals	A small blood pressure drop in hypertension5
Cholesterol	No LDL change at a clove a day2	No LDL change in the Stanford trial2
Main cautions	Burns if held on skin; garlic-in-oil safety	Bleeding, surgery, HIV drugs, pregnancy11214

Buying checklist, if you choose a supplement:

• Check for Allium sativum bulb and the form: powder, coated tablet, aged extract or oil¹⁰¹².
• Match the dose to what was studied for blood pressure, such as 1.2 grams of aged extract a day⁵.
• Ask who paid for the research a brand cites. Maker funding is common in garlic trials²⁶.
• Look for an independent quality seal, which tells you what’s in the bottle, not whether it works.
• Stop it a week before surgery and tell your surgeon¹².
• Give it 12 weeks, check your home blood pressure log, and decide with your clinician whether it’s worth continuing⁵.

Questions readers ask

The bottom line

Garlic deserves its place in kitchens around the world. As a supplement, it has one modest, real effect: a small drop in blood pressure for people with hypertension, with key trials funded by an extract maker. Stanford’s careful trial found no cholesterol benefit, and the cold evidence is a single small study.

Use garlic freely as food, crush it before you cook it, and keep it off your skin. Treat garlic pills like medicine: check them against your other drugs, stop them before surgery, and never use them in place of proven blood pressure or cholesterol care.

Related reading

• Turmeric is a superb spice with a shaky record as a pill
• Spices can help you cut salt, though some batches have carried lead
• Some healthy habits clash with your medicines, so check these goals first
• A prebiotic soda has 2 grams of fiber, and half an onion may have more
• Nature makes both medicine and deadly poison, and respecting it means telling them apart

Sources

1. National Center for Complementary and Integrative Health. “Garlic.” NIH, last updated February 2025. nccih.nih.gov
2. Stanford University School of Medicine. “Stanford Study Drives Stake Through Claims That Garlic Lowers Cholesterol Levels” (on Gardner CD, et al., Archives of Internal Medicine, 2007). February 26, 2007. med.stanford.edu
3. European Medicines Agency, Committee on Herbal Medicinal Products. “Assessment Report on Allium sativum L., Bulbus” (final). EMA/HMPC/7686/2013, July 18, 2017. ema.europa.eu
4. Ried K, Frank OR, Stocks NP, Fakler P, Sullivan T. “Effect of Garlic on Blood Pressure: A Systematic Review and Meta-Analysis.” BMC Cardiovascular Disorders, 2008;8:13 (University of Adelaide repository). digital.library.adelaide.edu.au
5. Ried K. “Garlic Lowers Blood Pressure in Hypertensive Subjects, Improves Arterial Stiffness and Gut Microbiota: A Review and Meta-Analysis.” Experimental and Therapeutic Medicine, 2020;19(2):1472-1478. spandidos-publications.com
6. NutraIngredients. “Aged Garlic May Help Reduce Blood Pressure, Boost Heart Health: Study” (on Ried K, Travica N, Sali A, Integrated Blood Pressure Control, 2016;9:9-21). February 29, 2016. nutraingredients.com
7. Lissiman E, Bhasale AL, Cohen M. “Garlic for the Common Cold.” Cochrane Database of Systematic Reviews, 2014, Issue 11. cochrane.org
8. Kim JY, Kwon O. “Garlic Intake and Cancer Risk: An Analysis Using the Food and Drug Administration’s Evidence-Based Review System for the Scientific Evaluation of Health Claims.” American Journal of Clinical Nutrition, 2009;89(1):257-264 (Ewha Womans University record). pure.ewha.ac.kr
9. NutraIngredients. “Garlic Science Lacking for Cancer Health Claims: Researchers.” January 7, 2009. nutraingredients.com
10. Seki T, Hosono T. “Functionality of Garlic Sulfur Compounds (Review).” Biomedical Reports, 2025;23:124. spandidos-publications.com
11. Song K, Milner JA. “The Influence of Heating on the Anticancer Properties of Garlic.” Journal of Nutrition, 2001;131(3s):1054S-1057S (abstract as reproduced by Chiro.org). chiro.org
12. European Medicines Agency, Committee on Herbal Medicinal Products. “Draft European Union Herbal Monograph on Allium sativum L., Bulbus.” EMA/HMPC/7685/2013, July 12, 2016. ema.europa.eu
13. HIV i-Base. “Interaction Between Garlic and Saquinavir and Underreporting of Alternative Treatment Use” (report on Piscitelli SC, et al., 8th Conference on Retroviruses and Opportunistic Infections, 2001). March 17, 2001. i-base.info
14. Borek C; reply by Falloon J, Gallicano KD, Burstein AH, Piscitelli SC. Correspondence on Piscitelli SC, et al., “The Effect of Garlic Supplements on the Pharmacokinetics of Saquinavir” (Clinical Infectious Diseases, 2002;34:234-238). Clinical Infectious Diseases, 2002;35:343 (University of Adelaide repository).
15. NIH Office of Dietary Supplements. “Dietary Supplements: What You Need to Know.” NIH, updated January 4, 2023. ods.od.nih.gov