Adverse interactions between warfarin and nonsteroidal antiinflammatory drugs: Mechanisms, clinical significance, and avoidance

OBJECTIVE: TO review the mechanisms and clinical significance of adverse interactions between warfarin and nonsteroidal antiinflammatory drugs (NSAIDs) and discuss how these interactions can be avoided. DATA SOURCES: Previous studies of interactions between warfarin and NSAIDs or reports of adverse interactions were identified from a MEDLINE search (1976 to present) and from the reference lists of pertinent articles. STUDY SELECTION AND DATA EXTRACTION: ALL articles were considered for inclusion in the review. Pertinent information was selected for discussion. DATA SYNTHESIS: All NSAIDs can prolong bleeding time by inhibiting platelet function. High-dose aspirin has a direct hypoprothrombinemic effect. Phenylbutazone and its analogs enhance the hypoprothrombinemic effect of warfarin through a pharmacokinetic interaction by inhibiting the hepatic metabolism of warfarin. Mefenamic acid also enhances the anticoagulant effect of warfarin, but the mechanism is not known. The clinical relevance of protein binding displacement in the interaction between warfarin and NSAIDs has been overstated, although a significant one may be more likely in the presence of high concentrations of NSAIDs in patients with slow elimination of warfarin (e.g., those with severe heart failure or impaired liver function). NSAIDs can induce gastrointestinal bleeding, which is likely to be more severe if warfarin is also given. CONCLUSIONS: The combined use of warfarin and NSAIDs is generally discouraged because of the increased risk of bleeding in these patients. In patients receiving warfarin who also require NSAIDs, phenylbutazone and its analogs, high-dose aspirin, mefenamic acid, excessive use of topical methyl salicylate, and NSAIDs that are associated with a higher risk of bleeding peptic ulcers should be avoided. Patients should be closely monitored for anticoagulant control and bleeding complications during the combined use of warfarin and NSAIDs.