Merck Manual

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Laura Shane-McWhorter

, PharmD, University of Utah College of Pharmacy

Reviewed/Revised Jan 2022 | Modified Sep 2022

What is garlic?

Garlic is an herb that has long been used in cooking and in medicine. When a garlic bulb is cut or crushed, an amino acid by-product called allicin is released. Allicin is responsible for garlic’s strong odor and medicinal properties.

Because the active ingredients are destroyed when the garlic is crushed, the amount of active ingredient in the various forms of garlic varies greatly. Aged garlic extract (AGE), made from garlic allowed to age for at least 20 months, has more stable active compounds than most forms. Consuming garlic supplements in this form appears to provide the greatest health benefits and with fewer adverse effects.

What claims are made about garlic?

Garlic reduces the normal clotting tendency of platelets (particles in the blood that help stop bleeding). Because garlic stops microorganisms (such as bacteria) from reproducing, it has some antiseptic and antibacterial effects. Proponents also claim that garlic can

Does garlic work?

The strongest evidence available for garlic supplementation, specifically AGE, is for lowering blood pressure. In large doses, garlic can reduce systolic and diastolic blood pressure slightly.

  • The effect of garlic on lowering cholesterol has been inconsistent, but studies have shown it lowers levels of total cholesterol and low-density lipoprotein (LDL)—the bad—cholesterol, probably enough to meaningfully decrease risks of heart problems such as heart attacks.

  • Scientific evidence for either garlic intake and garlic supplement use shows limited or no protection against cancer.

  • A meta-analysis of 7 studies shows that garlic may lower fasting blood sugar levels.

Most studies have used aged garlic extracts. Preparations formulated to have little or no odor may be inactive and need to be studied.

What are the possible side effects of garlic?

Garlic usually has no harmful effects other than making the breath, body, and breast milk smell like garlic. However, consuming large amounts can cause nausea and burning in the mouth, esophagus, and stomach.

What drug interactions occur with garlic?

Small amounts of garlic that may be eaten as part of the diet are unlikely to cause drug interactions. However, larger amounts as may be taken as supplements may interact with drugs that prevent blood clots (such as warfarin), increasing risk of bleeding. Thus, garlic should not be eaten or taken as a supplement 2 weeks before surgery or before a dental procedure.

Garlic may interact with drugs used to treat human immunodeficiency virus (HIV) infections (such as saquinavir) making them less effective, and may interact with drugs that decrease blood sugar levels Medication Treatment of Diabetes Mellitus Many people with diabetes require medication to lower blood glucose levels, relieve symptoms, and prevent complications of diabetes. There are two types of diabetes mellitus Type 1, in which... read more Medication Treatment of Diabetes Mellitus causing excessive decreases in blood sugar levels.

Garlic can also add to the effects of blood pressure–lowering drugs and thus lower blood pressure too much. Garlic can also interact with tacrolimus (a drug to prevent rejection of a transplanted organ), resulting in increased levels of tacrolimus and liver damage. Animal studies have shown that garlic decreases isoniazid levels.


Garlic appears to lower blood pressure, total cholesterol and LDL cholesterol levels, and fasting blood sugar levels. As a result, garlic may help decrease risks of cardiovascular problems. Garlic is relatively safe, but people should talk with their doctor before taking garlic if they

  • Have diabetes or HIV

  • Take anticoagulants

  • Take antihypertensives (for high blood pressure)

  • Take drugs to prevent organ transplant rejection

  • Take drugs to treat diseases such as tuberculosis

Drugs Mentioned In This Article

Generic Name Select Brand Names
No brand name available
Coumadin, Jantoven
Fortovase, Invirase
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