What it is
Garlic, a member of the lily family, has been prized for its healing properties since at least 2600 B.C., when ancient Sumerians chiseled prescriptions for the herb onto clay tablets. In cultures around the world, healers have embraced garlic as a salve for everything from colds to heart disease. Prior to the discovery of penicillin, garlic was the treatment of choice for infections such as pneumonia, bronchitis, tuberculosis, and dysentery.

Today, both traditional and alternative health practitioners value garlic particularly for its beneficial effects on the cardiovascular system. It is also widely recommended as a natural antibiotic for treating respiratory and other infections.

How it works
Sulfur compounds give garlic its characteristic odor and flavor and are believed to be responsible for its wide-ranging health benefits as well. Most of the research has focused on allicin, a sulfur compound with antimicrobial, antioxidant, cholesterol-lowering, and blood clot–preventing properties. Allicin is created when alliin, a sulfur-containing amino acid in garlic, comes into contact with another garlic compound, the enzyme allinase. This enzymatic reaction takes place when garlic is chopped, crushed, or chewed; it is destroyed during cooking. As with many herbs, garlic contains additional compounds that probably contribute to its health benefits.

Garlic appears to benefit the cardiovascular system in various ways. In a four-year study of 152 patients, German researchers found high doses of powdered garlic significantly reduced the buildup of atherosclerotic plaque and even slightly reduced existing plaque (Atherosclerosis, 1999, vol. 144, no. 1). In another study published in the journal Circulation, researchers found that standardized garlic powder helped maintain aortic elasticity (1997, vol. 96, no. 8). Aortic stiffness commonly occurs with aging and high blood pressure, and it is a significant risk factor in cardiovascular disease.

Garlic outperforms a placebo for decreasing cholesterol levels, but the reduction is generally modest, according to researchers who have analyzed decades of studies. And though garlic has long been used in folk medicine to lower blood pressure, research doesn't support this application.

A well-designed clinical trial done in 2001 strongly supports the use of garlic for preventing and treating colds. In the study, half of a 146-volunteer group received a garlic supplement containing allicin; the other half took a placebo. During a 12-week period between November and February, the volunteers recorded cold symptoms in a daily diary. At the end of the study, researchers found that the garlic group reported 24 colds, compared with the 65 colds suffered by the placebo group. The researchers also discovered that the garlic takers who did get sick recovered more quickly than those not taking garlic (Advances in Therapy, 2001, vol. 18, no. 4).

Side effects
Raw garlic can cause gastrointestinal upset if taken on an empty stomach, so it's best to add it to meals. Sprinkle chopped raw garlic onto pasta, soups, and salads, or spread it onto toasted bread with olive oil. The most common side effect associated with garlic is, of course, garlic breath, which can be somewhat alleviated by chewing fresh parsley or fennel seeds.

Garlic appears to have no effect on drug metabolism, but patients taking anticoagulants should consult a doctor before using medicinal amounts of garlic because of the herb's blood-thinning effect. For the same reason, it's probably wise to discontinue garlic supplements seven to ten days before surgery.

How to take it
A great deal of disagreement exists over the best form and dose of garlic. According to the Herb Research Foundation, a typical garlic dose is 600 to 900 mg per day of powdered garlic in capsules or tablets (standardized for alliin content); 4 ml per day of aged garlic liquid extract; 10 mg per day of garlic oil capsules; or one medium-size clove of fresh garlic per day.

Herbalist and author Laurel Vukovic lives in Ashland, Oregon, and has published nine books, including Herbal Healing Secrets for Women (Prentice Hall, 2000).