Garlic

Garlic’s use dates back thousands of years as both a traditional medicine and seasoning. Some chemicals in it are thought to confer health benefits—notably allicin, which gives garlic its strong taste and smell, though not all scientists agree that it is the main beneficial ingredient. Garlic supplements vary widely in composition, depending on the age of the garlic and how it is processed.

Claims, purported benefits: Lowers cholesterol; fights cancer, hypertension, diabetes, and infections, including the common cold.

Evidence: Lab and animal studies suggest that garlic has a range of benefits. It keeps blood platelets from sticking together (reducing the risk of blood clots) and may have anti-cancer and cholesterol-lowering effects. But what happens in people is less clear. Most human studies have been small, short, and inconsistent. Some have found that garlic reduces cholesterol by about 10%, but a well-designed Stanford study in 2007 found that neither raw garlic nor supplements had an effect.

Bottom line: There’s no clear evidence that garlic pills are beneficial. Even if they lower blood cholesterol, the effect is relatively small, especially compared to a heart-healthy diet and medication. And no one knows what dose or form—powder, oil, or aged "deodorized" extracts, for example—would be best. Supplements may increase the risk of bleeding if taken with warfarin (Coumadin), aspirin, or possibly fish-oil pills, and they may interact with medications, including certain ones for diabetes and hypertension. Some may cause nausea, heartburn, bad breath, and body odor. In independent testing, many garlic supplements did not meet their label claims or were contaminated with lead. But there’s no harm in eating more garlic—though cooking garlic at high temperature destroys potentially active components.