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Carbamazepine and Contraception — MRCPsych Paper B MCQ

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ModerateMood DisordersCarbamazepine and ContraceptionMRCPsych Paper B

A 32-year-old woman with bipolar I disorder is established on carbamazepine for mood stabilisation. She wishes to start a standard-dose combined oral contraceptive pill. What is the main clinical concern about using these medicines together?

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Correct answer: D — Carbamazepine induces hepatic enzymes, reducing contraceptive steroid exposure

Carbamazepine is a potent hepatic enzyme inducer, including induction of CYP3A4. It increases the metabolism of contraceptive steroids, reducing their exposure and potentially causing breakthrough bleeding, ovulation and unintended pregnancy. Combined hormonal contraception is therefore generally not advised during ongoing carbamazepine treatment; an effective method unaffected by enzyme induction should be discussed, such as an intrauterine method or depot medroxyprogesterone acetate. DMPA does not require a shortened dosing interval for this interaction. The combined pill does not characteristically inhibit carbamazepine metabolism sufficiently to cause toxicity, and carbamazepine does not enhance contraceptive efficacy or increase steroid exposure. Neither medicine has a relevant serotonergic interaction, so serotonin syndrome is not expected.

Reference: Electronic Medicines Compendium (UK), Tegretol 400 mg Prolonged Release Tablets, Summary of Product Characteristics, sections 4.4–4.6 (special warnings and interactions: hormonal contraceptive failure). https://www.medicines.org.uk/emc/product/7843/smpc