St. John's wort (Hypericum perforatum) is one of the best-selling herbal remedies in the United States. It has been implicated as an inducer of the P450 enzyme system, and as such, may cause increased metabolism of certain drugs, including oral contraceptives. Women using oral contraceptives have been warned against using St. John's wort. To date, there are some case reports but little clinical data demonstrating risk of contraceptive failure if they do. This article reviews available data and discusses theoretical reasons for concern about possible drug-herb interactions.