Rifampin is a potent inducer of hepatic cytochrome P450 enzymes. When co-administered with warfarin, what is the expected pharmacokinetic consequence?

• A Decreased warfarin plasma concentration and reduced anticoagulation ✓
• B Increased warfarin plasma concentration and bleeding risk
• C No change in warfarin levels as it is renally excreted
• D Increased warfarin half-life due to enzyme inhibition

Explanation

Enzyme inducers like rifampin increase the synthesis of CYP enzymes (especially CYP3B4 and CYP2C9), accelerating the metabolism of substrates such as warfarin. This decreases the plasma concentration and pharmacological effect of warfarin, risking therapeutic failure. Enzyme induction takes days to weeks as new protein must be synthesized. This is a major drug interaction tested in Indian PG entrance exams.

Reference: Katzung Basic and Clinical Pharmacology, 15th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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