A 55-year-old man on long-term cimetidine for reflux oesophagitis develops gynaecomastia and impotence. The underlying property of cimetidine responsible is:

• A Inhibition of hepatic CYP450 enzymes causing hyperoestrogenaemia
• B Direct stimulation of breast tissue H2 receptors
• C Blockade of hypothalamic dopamine D2 receptors raising prolactin
• D Anti-androgenic activity at the androgen receptor ✓

Explanation

Cimetidine binds androgen receptors as a weak antagonist and also inhibits the metabolism of oestradiol, producing gynaecomastia, impotence and loss of libido, particularly in older men on high doses. Ranitidine, famotidine and nizatidine lack this anti-androgenic action, which is the usual reason cited for switching therapy. Cimetidine also inhibits several CYP450 enzymes, but this causes drug interactions such as raised warfarin levels rather than the endocrine effects described here.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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