MAOI Washout Period — MRCPsych Paper B MCQ

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ModerateMood DisordersMAOI Washout PeriodMRCPsych Paper B

A 42-year-old woman with treatment-resistant depression is being considered for treatment with phenelzine. Her current antidepressant is mirtazapine, which is tapered and discontinued. What is the minimum interval that should elapse after her final dose of mirtazapine before phenelzine is started?

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Correct answer: E — 2 weeks

The correct answer is E: 2 weeks. The mirtazapine SmPC states that about two weeks should pass after mirtazapine is stopped before an MAO inhibitor is started. This washout reduces the risk of a serious pharmacodynamic interaction, including serotonin syndrome. A direct switch or a one-week interval is therefore insufficient. The required interval depends on the antidepressant being discontinued rather than being identical for every drug. Five weeks is a recognised longer washout before phenelzine when switching from fluoxetine because fluoxetine and its active metabolite have prolonged elimination; it is not required for mirtazapine. Six weeks would also be unnecessarily long in this case. The interval is measured from the final mirtazapine dose after any clinically appropriate taper.

Reference: Electronic Medicines Compendium, Mirtazapine 30 mg tablets, Summary of Product Characteristics, section 4.5 Pharmacodynamic interactions, updated 2025. https://www.medicines.org.uk/emc/product/100083/smpc