In recent years, much evidence for alteration of the anticoagulant activity of oral courmarin and indandione anticoagulants by interacting drugs has appeared in the literature. This review article summarizes clinical reports of drugs and pathological states that have been clinically shown to interfere with the activity and efficacy of these oral anticoagulants. Tables of drugs altering prothrombin time and drugs affecting hepatic function are presented along with theoretical implications of such phenomena on the activity of oral anticoagulants. The importance of possible interactions by drugs similar in configuration and chemical structure to known interactors is emphasized. Care is taken to separate clinical evidence of interactions from possible interactions from a theoretical point of view. Possible mechanisms for the tabulated drug interactions are included. Although the mechanisms are largely hypothetical, they may illuminate the significance and true nature of the interactions.