What Is Garlic?

Medically Reviewed

Garlic’s use dates back thousands of years as both a traditional medicine and a seasoning. Traditionally, garlic has been an integral part of the Mediterranean diet and many Asian cuisines. Garlic supplements—powders, oils, or aged extracts—have become popular, especially for people who don’t like the taste of garlic but want its potential health benefits.

Claims, purported benefits: Lowers cholesterol; helps prevent or treat hypertension, diabetes, cancer, arthritis, Alzheimer’s disease, and infections, including the common cold.

What the science says: Garlic contains many interesting compounds that have been linked to a host of proposed health benefits. One of the key components is allicin, a sulfur compound that is formed in raw garlic after a clove is cut or crushed. Allicin is the major source of the bioactive compounds that provide garlic’s strong taste and smell. But not all scientists agree that allicin itself is the main beneficial ingredient, since it breaks down quickly into other compounds. In fact, no one knows which, if any, component is most important; different ones may have different effects in the body.

Lab and animal studies suggest that garlic (or isolated compounds from it) has a range of benefits. For example, it keeps blood platelets from sticking together, which reduces the risk of blood clots, and may have anti-cancer, anti-inflammatory, antioxidant, and cholesterol-lowering effects. But what happens in people is still unclear. Nearly all human studies have been small, short, and/or poorly designed. Moreover, they have used different garlic preparations and doses, making comparison difficult.

Cholesterol. While some studies (many using supplements) have found that garlic reduces LDL (“bad”) cholesterol slightly, others have shown little or no effect. A well-designed 2007 study from Stanford University found no benefit when it tested raw garlic and two popular supplements (one containing powdered garlic, the other an aged-garlic extract) for six months in people with high LDL. Several analyses concluded that good clinical trials have not shown consistent or significant improvements, but an Australian analysis of 39 clinical trials, published in Nutrition Reviews in 2013, concluded that garlic can reduce LDL modestly when taken for at least two months. Regardless of its effect on cholesterol, there has been no evidence that garlic prevents heart attacks.

Blood pressure. Garlic supplements can lower blood pressure modestly in people with hypertension, accord- ing to a 2015 analysis of 17 short-term clinical trials, in the Journal of Clinical Hypertension. But various prepara- tions may have different effects on blood pressure.

Cancer. The evidence is mixed at best. Some (not all) population studies have found that people who eat a lot of garlic have a lower risk of certain cancers. But there have been few large, long-term random- ized trials, which are needed to prove that it’s really garlic, and not something else about garlic eaters, that affects cancer risk. Two such studies done in China reached opposite conclusions about the effect of garlic supplements on the risk of stomach cancer. A 2013 Harvard study found no association between garlic (food or supplements) and colon cancer.

Colds. Despite a common belief that garlic can prevent colds, there has been remarkably little human research on this. In 2012, a study in Clinical Nutrition found that an aged garlic extract taken for three months did not reduce the incidence of colds or flu but did reduce their severity.

Special precautions: Garlic supplements vary widely, depending on the age of the garlic and how it is processed. There’s debate about which form (powder, oil, or aged “deodorized” garlic extract, for example), if any, is best. There is no accepted standard dose. Some products give “alliin” amounts; alliin is the substance that is converted to allicin. Claims such as “allicin-rich” or “high potency” don’t mean much. Testing of garlic supplements has found that nearly half have problems.

Common side effects: Some garlic supplements may reduce blood clotting, which could theoretically be a problem if you have a bleeding disorder, are planning to have surgery, or are already taking a drug that affects blood clotting, such as warfarin. The supplements may interact with some medications for diabetes, HIV disease, hypertension, cancer, and cholesterol. Like raw garlic, supplements may cause nausea, heartburn, stomach upset, bad breath, and body odor.

Our advice: As a Chinese research review in Critical Reviews in Food Science and Nutrition in 2013 concluded, there is no consistent evidence from well-designed trials indicating that garlic supplements are beneficial. Even if they do lower blood cholesterol or blood pressure or help prevent blood clots—all of which is uncertain—the effect is small, so the supplements can’t replace medication. In any case, no one knows what form or dose would be best.