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 are marketed to lower cholesterol, as well as fight cancer, hypertension, diabetes, arthritis, Alzheimer’s disease and infections, including the common cold.
Garlic: allicin wonderland
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 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 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 and cholesterol-lowering effects.
But what happens in people is unclear. Nearly all human studies have been small, short and/or poorly designed. Plus, they have used different garlic preparations and doses, making comparison difficult.
Here are some main areas of interest:
- 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. More recently, two analyses concluded that good clinical trials have not shown consistent or significant improvements. Regardless of its effect on cholesterol, there’s no evidence that garlic actually prevents heart attacks.
- Blood pressure: Some small, short-term studies have found that garlic can lower blood pressure slightly in people with hypertension. But various garlic preparations may have different effects on blood pressure.
- Cancer: The evidence is mixed, at best. Some, but 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 randomized 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 pills on the risk of stomach cancer.
- Colds: Despite a common belief that garlic can prevent colds, there has been remarkably little human research on this. In 2012, a study published 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 when they occurred.
- Other claims: For other conditions, such as upper respiratory infections, diabetes and arthritis, there’s no good evidence of benefit.
Garlic: supplemental problems
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. But claims such as “allicin-rich” or “high potency” don’t mean much. Testing of garlic supplements has found that nearly half have problems.
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 taking a drug that affects blood clotting, such as warfarin (Coumadin).
The supplements also may interact with some medications for diabetes, HIV, hypertension, cancer and high cholesterol. Their effect on many drugs has not been studied. Like raw garlic, supplements may cause nausea, heartburn, stomach upset, bad breath and body odor.
Bottom line: There is no clear evidence that garlic supplements are beneficial. Even if they do lower blood cholesterol or blood pressure or thin the blood, 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. Still, there’s no harm in eating more garlic in your food, if you like it.
Originally published January 2012. Updated February 2014.