trimethoprim + sulfamethoxazole + warfarin

This is a combination of three drugs—trimethoprim, sulfamethoxazole, and warfarin—administered together. Trimethoprim and sulfamethoxazole are antibiotics that act synergistically to inhibit sequential steps in bacterial folate synthesis, making the combination effective against a broad range of bacterial infections. Warfarin is an oral anticoagulant that inhibits vitamin K epoxide reductase, thereby reducing the synthesis of active clotting factors II, VII, IX, and X. The co-administration of trimethoprim/sulfamethoxazole with warfarin is clinically significant due to pharmacokinetic interactions; specifically, sulfamethoxazole inhibits CYP2C9-mediated metabolism of S-warfarin (the more potent enantiomer), leading to increased anticoagulant effect and heightened risk for bleeding complications[3][6][8]. This interaction necessitates close monitoring or dose adjustment when these drugs are used together.