A patient stabilized on warfarin starts self-medication with an over-the-counter ulcer remedy. Three days later his INR rises sharply without any change in warfarin dose. Which histamine H2 receptor blocker most likely caused this interaction?

• A Ranitidine
• B Cimetidine ✓
• C Nizatidine
• D Famotidine

Explanation

Cimetidine contains an imidazole ring and potently inhibits several cytochrome P450 isoenzymes, reducing the metabolism of drugs such as warfarin, phenytoin, theophylline and diazepam. Ranitidine, famotidine and nizatidine have far weaker or negligible CYP inhibition, so they are preferred when a patient needs an H2 blocker alongside narrow therapeutic index drugs. Cimetidine also has antiandrogenic effects causing gynecomastia, another distinguishing adverse profile.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.