A 70-year-old man on cimetidine for dyspepsia develops gynaecomastia and impotence, and his warfarin INR rises sharply after starting the same drug. Which properties of cimetidine explain both findings?

• A Antiandrogenic activity and inhibition of cytochrome P450 enzymes ✓
• B Hyperprolactinaemia and induction of CYP2C9
• C Blockade of gastric H2 receptors in breast tissue and renal tubular secretion of warfarin
• D Competitive displacement of warfarin from albumin and estrogen receptor agonism

Explanation

Cimetidine binds peripheral androgen receptors, producing gynaecomastia, impotence and reduced sperm count, particularly in older men on high doses. It is also a potent inhibitor of several cytochrome P450 isoforms including CYP1A2, CYP2C9 and CYP3A4, raising levels of warfarin, phenytoin, theophylline and others. Famotidine is essentially free of both effects, which is why it is the preferred H2 blocker when these risks matter.

Reference: Katzung's Basic and Clinical Pharmacology, 16th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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