The use of herbal medications and other alternative therapies is accelerating. Survey data clearly indicate that these agents are frequently combined with prescription and over-the-counter medications. The herbal antidepressant St. John's wort (Hypericum perforatum) is one of the most commonly utilized herbal agents. In spite of growing concern and examples of herb-drug interactions, little systematic research has been published or funded in this area. Computerized searches of the biomedical literature were undertaken utilizing MEDLINE, Current Contents, and PsycINFO computer databases (years 1966-December 2000) and by review of bibliographies to identify all pertinent case reports, case series, and formal studies for this review using search terms St. John's wort, hypericum, herb, in vitro, cytochrome P450, and drug interactions. Little in vitro or in vivo data on St John's wort or other herb-drug interactions is available and current in vitro methods for screening conventional medications may have limited applications to herbal agents which generally have numerous constituents of unknown pharmacokinetics and pharmacology. However, available data from clinical studies and case reports suggests that St. John's wort is unlikely to inhibit cytochrome P450 (CYP) 3A4 or 2D6, but is likely an inducer of CYP 3A4 and possibly the P-glycoprotein transporter. Examples of conventional medications which may undergo significant CYP 3A4 induction by St. John's wort include cyclosporine, indinavir, and oral contraceptives. The accumulating evidence of significant drug interactions with St. John's wort should serve as an example to clinicians to be aware of the potential for St. John's wort, and very likely, other herbal products to participate in important herb-drug interactions when used in combination with conventional medications. Concomitant use of herbal agents and conventional medications should generally be discouraged until further information is available. Additional research is urgently needed in this area.