Neostigmine Duration of Action — FRCA Primary MCQ
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Correct answer: D — 60–90 minutes
The best answer is D: approximately 60–90 minutes. Neostigmine is a quaternary ammonium, reversible acetylcholinesterase inhibitor. It carbamylates the enzyme’s esteratic site; hydrolysis of this carbamylated complex is slower than hydrolysis of the normal acetyl-enzyme intermediate, producing an intermediate duration of action. Its pharmacokinetic profile after intravenous administration is consistent with activity persisting for roughly an hour or longer. A 5–10-minute duration is more characteristic of the short-acting anticholinesterase edrophonium. The 20–30-minute option may be confused with a reported elimination half-life, but plasma half-life and pharmacodynamic duration are not interchangeable. Pyridostigmine has a longer action, commonly several hours, making 2–4 hours a plausible distractor. Six to eight hours is too prolonged for intravenous neostigmine.
Reference: ADVANZ Pharma. Glycopyrronium Bromide and Neostigmine Metilsulfate 0.5 mg/2.5 mg per ml Solution for Injection, SmPC sections 5.1–5.2. Revised 2024. https://www.medicines.org.uk/emc/product/6599/smpc