Access to information on clinically significant drug interactions is not readily available. This can be a source of uncertainty, and locating this information can be tedious and time-consuming. The pharmacology of drug interactions is briefly discussed. The most common drug interactions involve altered hepatic metabolism. At least 26 drugs are such frequent offenders that they can be considered “red flag” drugs. In addition, an extensive list of current significant and less significant drug interactions of particular importance to the dermatologist is presented. (J Am Acad Dermatol 1998;38:599-612.)