Allicin
Allicin is a volatile oil found in the bulb of the garlic plant, and is the compound giving garlic its distinctive odor. Allicin and the other volatile sulfur compounds are considered to be responsible for the biological activity of garlic oil. Some of these biological activities include reduction of serum cholesterol and of blood sugar levels, as well as lowering blood pressure. Allicin also has antifungal and antibacterial activities.
References
Leung, Albert Y. 1980. Encyclopedia of Common Natural Ingredient used in Food, Drugs and Cosmetics. John Wiley and Sons, N Y. 409 pp.
Windholz, Martha, Susan Budavani, et. al. 1976. The Merck Index. Merck and Co., Inc. New Jersey.