Merck Manual

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Laura Shane-McWhorter

, PharmD, University of Utah College of Pharmacy

Last full review/revision Jul 2020| Content last modified Jul 2020
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Garlic 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.

Medicinal claims

Garlic reduces the normal clotting tendency of particles in the blood that help stop bleeding (platelets). Because garlic stops microorganisms (such as bacteria) from reproducing, it can be used as an antiseptic and antibacterial. 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. Claims that garlic helps prevent cancer are supported by little evidence. 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.

Possible side effects

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.

Possible drug interactions

Garlic 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 one week before surgery or before a dental procedure. Garlic may interact with drugs used to treat human immunodeficiency virus (HIV) infections making them less effective, and may interact with drugs that decrease blood sugar levels causing excessive decreases in blood sugar levels.

More Information

The following is an English-language resource that may be useful. Please note that THE MANUAL is not responsible for the content of this resource.

NOTE: This is the Consumer Version. DOCTORS: Click here for the Professional Version
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