Garlic Extract Supplements: Allicin, Aged Garlic, and What They Do

Stop garlic supplements 1 to 2 weeks before surgery (Memorial Sloan Kettering), and talk to your prescriber first if you take warfarin or other blood thinners, aspirin, cyclosporine, saquinavir or darunavir, or insulin.

What is garlic extract?

Garlic has been used as food and medicine for thousands of years, with ancient texts from Egypt, Greece and India describing it for headaches, throat trouble and digestive complaints (NCCIH). Today garlic supplements are promoted for cholesterol, blood pressure, blood sugar and immune support.

"Garlic extract" can mean very different products. Some are dried garlic powder pressed into tablets. Others are garlic soaked for many months, oils pulled out by steam, alcohol tinctures, or whole cloves aged until they turn black. Each process changes which sulfur compounds end up in the bottle, and that changes what the research says about it.

Allicin: how garlic makes its key compound

Inside an intact clove, the cells hold alliin, an odorless sulfur compound, while a separate enzyme called alliinase sits in neighboring cells. Crushing or chopping the clove lets the two meet, and alliin turns into allicin, the sharp, pungent compound behind garlic's bite. Allicin is reactive and short-lived, and it quickly breaks down into other sulfur compounds (Memorial Sloan Kettering).

Many experts treat a product's ability to form allicin as the best single measure of garlic activity. That creates a puzzle for supplements. Dried garlic powder still contains alliin and alliinase, and adding water forms allicin in about half a minute. But alliinase stops working at a pH of about 3.5 or lower, a range common in the stomach. That's why many garlic tablets have an enteric coating, meant to hold the tablet together until it reaches the less acidic small intestine (Lawson and Hunsaker, Nutrients, 2018).

Does the coating help? Lawson and Hunsaker tested 13 supplements and 9 garlic foods in 13 volunteers by tracking a breath marker of allicin. Their results surprised them:

• Enteric tablets delivered 36 to 104 percent of the allicin of crushed raw garlic, but only 22 to 57 percent when taken with a high-protein meal, which slows stomach emptying.
• Uncoated tablets did well regardless of the meal, at 80 to 111 percent.
• Garlic powder capsules ranged widely, from 26 to 109 percent.
• Cooked garlic still produced some allicin-equivalent activity: 16 percent boiled, 30 percent roasted, 19 percent pickled. Black garlic gave 5 percent.

The takeaway: a coating isn't a guarantee, and the label's allicin number tells you what a tablet could produce, not exactly what your body gets.

Types of garlic supplements compared

Garlic powder tablets. Dried, ground cloves that keep their alliin and alliinase, so they form allicin once wet. Labels often list "allicin potential" or "allicin yield." Kwai garlic powder tablets, used in many clinical trials, delivered about 80 percent of raw garlic's allicin in the Lawson study.

Aged garlic extract. Sliced raw garlic left in a 20 percent alcohol solution for up to 20 months at room temperature. S-allyl cysteine (SAC) ends up as its main sulfur compound, with no active alliinase and little allicin-derived material left (Lawson and Hunsaker, 2018). Because SAC is stable, doses can be standardized, which is one reason researchers have favored it for blood pressure trials (Ried et al., European Journal of Clinical Nutrition, 2013).

Garlic oil. Steam-distilled garlic oil is made up of allyl sulfides formed after allicin breaks down. Memorial Sloan Kettering notes that garlic essential oil contains no allicin or ajoene, compounds linked to garlic's effects on cholesterol and blood clotting.

Black garlic. Whole bulbs held at 60 to 90 °C and high humidity for about 30 to 40 days. Browning reactions turn the cloves black and sweet, and the sharp allicin taste disappears (Lawson and Hunsaker, 2018). Heat processing leaves black garlic with more SAC and other water-soluble antioxidants than fresh garlic. Much of the research behind its reported benefits is preclinical, and more studies are under way (Ahmed and Wang, Molecules, 2021).

"Odorless" garlic. Deodorized extracts or coated tablets designed to cut garlic breath. Lawson and Hunsaker found that "odor-controlled" or "reduced odor" describes most products more accurately than "odorless."

Garlic extract and blood pressure

Blood pressure is where garlic supplements have the most consistent results. A 2016 meta-analysis of 20 trials with 970 participants found garlic lowered systolic pressure by about 5 mm Hg and diastolic by about 2.5 mm Hg compared with placebo. In trials limited to people with high blood pressure, the drops were larger: about 8.7 mm Hg systolic and 6.1 mm Hg diastolic (Ried, Journal of Nutrition, 2016).

A 12-week trial of aged garlic extract enrolled 79 people whose blood pressure stayed high despite medication. Two capsules a day (480 mg of extract with 1.2 mg of SAC) lowered systolic pressure by about 11.8 mm Hg more than placebo. One capsule didn't make a clear difference, and four capsules caused more side effects (Ried et al., 2013).

NCCIH is more guarded and calls the evidence limited, with only small reductions. Garlic is not a replacement for blood pressure medicine. If you take it alongside your prescriptions, keep checking your readings and tell your prescriber.

Garlic and cholesterol

A meta-analysis of 39 trials found garlic lowered total cholesterol by about 17 mg/dL and LDL cholesterol by about 9 mg/dL in people whose total cholesterol was above 200 mg/dL, but only when they took it for more than two months. HDL rose only slightly, and triglycerides didn't change meaningfully (Ried et al., Nutrition Reviews, 2013). NCCIH agrees the effect is small.

For comparison with other options people ask about, see our guides to red yeast rice and omega-3s.

Does garlic help with colds?

Not convincingly. A Cochrane review found only one trial good enough to include. In it, 146 people took either a garlic supplement (180 mg of allicin) or a placebo daily for 12 weeks. The garlic group reported 24 colds versus 65, though people recovered in about the same time once sick. With a single trial based on self-reported colds, the reviewers judged the evidence insufficient (Lissiman et al., 2014). NCCIH similarly notes that a 2022 review found only two small, flawed studies.

NCCIH adds that garlic may lower blood sugar slightly in people with diabetes, and that eating garlic doesn't appear to reduce stomach cancer risk.

Side effects and drug interactions

Garlic taken by mouth has been used safely in studies lasting up to seven years (NCCIH). Common side effects are bad breath and body odor, stomach pain, gas and nausea. In the aged garlic trial, a few people noticed bloating, reflux or a garlic taste in the first week. Some people are allergic to garlic.

• Bleeding: garlic supplements may raise bleeding risk. Tell your care team, especially before surgery or if you take anticoagulants or aspirin (NCCIH).
• HIV medicines: in 10 healthy volunteers, three weeks of garlic caplets cut blood levels of the protease inhibitor saquinavir by about half. Ten days after stopping garlic, levels had only recovered to 60 to 70 percent of baseline (Piscitelli et al., Clinical Infectious Diseases, 2002). Memorial Sloan Kettering also describes two cases where darunavir stopped working in people taking garlic.
• Transplant and diabetes drugs: garlic can reduce the effect of cyclosporine, and insulin doses may need adjusting (Memorial Sloan Kettering).
• Raw garlic on skin: fresh garlic applied to the skin can cause severe irritation and chemical burns (NCCIH).
• Pregnancy and breastfeeding: supplement amounts above what's found in food may not be safe (NCCIH).

Find it at William's

We carry several garlic forms so you can match the product to what you're after:

• Nature's Way Garlinase 5000, 100 tablets: an enteric-coated garlic tablet labeled to deliver 5,000 mcg of allicin. Take it as directed on the label, since meals can affect how coated tablets release.
• Now odorless garlic, 250 softgels: a purified, deodorized garlic extract made from whole cloves, for people who want garlic with less breath odor.
• Herb Pharm garlic extract, 1 oz: a liquid extract of fresh, cured (not dried) certified organic garlic bulbs, made with organic cane alcohol and taken in water.
• HealthAid fermented black garlic: whole cloves aged under controlled heat and humidity until black and sweet. They're ready to eat or can go into salads and recipes.

For another member of the allium family, see our onion guide.

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