Also known as the “stinking rose,” garlic is one of many plants in the allium family, which also includes chives, leeks, onions and scallions. Although grocery stores generally carry only one or two types of raw garlic, there are actually hundreds of sub-varieties that vary in size, shape, color, taste and other qualities. Garlic is also available as garlic powder and garlic salt, both of which are used in many recipes.

While garlic may be an important part of many culinary traditions, millions of people also take garlic preparations for their supposed health benefits. When garlic is pressed or crushed, an enzyme (which facilitates chemical reactions) called alliinase is activated. Alliinase helps to change the inactive compound alliin into the active compound allicin. Although allicin was once given the credit for garlic’s healthy effects, researchers now believe that allicin quickly changes as soon as it is exposed to oxygen in the air. Allicin combines with oxygen (oxidizes) to create new compounds, which results in over 75 active sulfur-containing substances such as S-allylcysteine and ajoene. The presence of these sulfur-containing substances earned garlic its classification as an organosulfur, and they now appear to be responsible for garlic’s health benefits, as well as its distinctive odor.

Traditionally, garlic has been used as a digestive aid and to fight infections. In more recent years, garlic has also been touted for its ability to lower cholesterol and high blood pressure, slow the advance of atherosclerosis, and prevent stomach and colon cancers. Medicinal garlic is usually sold in capsules or tablets that offer extracts of garlic. Some brands of garlic tablets are enteric coated, meaning they pass through the stomach for absorption by the small intestine to avoid any gastrointestinal discomfort.

Garlic's actual health benefits are the subject of wide-ranging research efforts. According to the National Center for Complementary Medicine, garlic may be able to slow the progression of atherosclerosis, and evidence is mixed on its ability to help lower high blood pressure. Garlic is most often mentioned for its ability lower cholesterol. However, in 2007, the results of one of the best-designed human studies to date on garlic and cholesterol were released. This study compared three formulations of garlic in a head-to-head test to see which, if any, was most effective at lowering cholesterol levels. Ultimately, researchers concluded that none of the garlic preparations were effective at lowering cholesterol.

Garlic is sometimes mentioned for its ability to reduce the risk of stomach and colon cancer, a claim that has not been validated by human trials.

Nevertheless, there remains an aggressive effort by supplement companies and some health experts to promote garlic as a healing herb. In many cases, pro-garlic advocates cite scientific studies, which may lead to a sense of confusion over garlic's health properties.

At least some of this confusion must be attributed to the problems with studying any herbal supplements, as well as a thorough understanding of the difference between lab studies and human studies. Supplements such as garlic are hard to study because it is hard to locate standardized extracts that are chemically identical, and because natural garlic contains many chemicals, it is difficult or impossible to know which chemicals may be causing any effect. Additionally, much of the research on garlic has been in lab studies, or studies in which scientists experiment on prepared solutions in test tubes, or animal studies. Garlic compounds that might be effective in the lab or animals may act differently once they are introduced into the human body.

It is unlikely these issues will be solved in the near future, at least in part because of the government's stance toward herbal and dietary supplements. These products are not under the control of the Food and Drug Administration, and while there are laws covering what supplement manufacturers are allowed to say in their advertising, there are no laws governing the purity, strength and production of garlic supplements. Unlike pharmaceutical products, so-called nutraceuticals are not required to undergo rigorous clinical testing in human beings to verify their health benefits. Fortunately, it appears there are few side effects to garlic supplements, so under the supervision of a physician, they can probably be safely added to most people's daily routine. The largest exception may be people who are taking anti-platelet drugs or who are scheduled for surgery, because garlic also has anti-platelet effects.