Which antihistamine is used as an antidote in mild serotonin syndrome because it antagonises 5-HT2A receptors centrally?

• A Chlorpheniramine
• B Fexofenadine
• C Hydroxyzine
• D Cyproheptadine ✓

Explanation

Cyproheptadine is a first-generation H1 antihistamine with strong central 5-HT2A antagonism, which forms the basis of its off-label use in serotonin syndrome after supportive care and benzodiazepines. Chlorpheniramine and hydroxyzine are essentially pure H1 blockers without meaningful antiserotonin activity, and fexofenadine does not cross the blood brain barrier at all. The same 5-HT2 blocking property explains cyproheptadine's older use in carcinoid diarrhoea and as an appetite stimulant.

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

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