TMP-SMX can interact with warfarin. What is the clinical effect?

TMP-SMX can interact with warfarin. What is the clinical effect?

• A. Increases warfarin effect, raising INR
• B. Decreases warfarin effect
• C. No interaction
• D. Increases risk of leukopenia

Answer: (A)

TMP-SMX can interact with warfarin by increasing its anticoagulant effect, which raises the INR and bleeding risk. This happens because trimethoprim inhibits enzymes (notably CYP2C9) that metabolize the more potent S-warfarin, leading to higher warfarin levels. Additionally, antibiotics can reduce gut vitamin K–producing bacteria, further enhancing anticoagulation. Clinically, starting TMP-SMX in a patient on warfarin requires closer INR monitoring and possible adjustment of the warfarin dose to prevent bleeding. The other options don’t fit this scenario: the interaction does occur and it enhances, not reduces, warfarin effect, and leukopenia is not the primary concern here.

TMP-SMX can interact with warfarin by increasing its anticoagulant effect, which raises the INR and bleeding risk. This happens because trimethoprim inhibits enzymes (notably CYP2C9) that metabolize the more potent S-warfarin, leading to higher warfarin levels. Additionally, antibiotics can reduce gut vitamin K–producing bacteria, further enhancing anticoagulation. Clinically, starting TMP-SMX in a patient on warfarin requires closer INR monitoring and possible adjustment of the warfarin dose to prevent bleeding. The other options don’t fit this scenario: the interaction does occur and it enhances, not reduces, warfarin effect, and leukopenia is not the primary concern here.