Recent case reports and studies suggest that interactions involving the cytochrome P-450 mixed function oxidase system are important causes of medication toxicity and de-creased efficacy during combination drug therapy. The cytochrome P-450 3A3/4 isoenzyme is involved in many significant drug interactions. New and familiar drugs continue to be implicated as having potentially serious interactions with this group of enzymes. An understanding of the basic principles of these interactions may have a major impact on patient outcome. (J Am Acad Dermatol 1997;37:765-71.)