Which drug decreases warfarin effect (↓INR)?

• A. Fluconazole
• B. Rifampin
• C. Metronidazole
• D. Bactrim

Answer: (B)

Warfarin effect is driven by its metabolism in the liver, and many drugs alter this by changing enzyme activity. When a drug induces hepatic enzymes, it speeds up how fast warfarin is cleared, lowering its plasma level and reducing the anticoagulant effect, which shows up as a lower INR. Rifampin is a strong inducer of hepatic enzymes, so it decreases warfarin's effect and often requires higher doses of warfarin with closer INR monitoring.

The other drugs listed tend to increase warfarin exposure by inhibiting its metabolism or displacing it from binding, which raises the INR and bleeding risk rather than lowering it.