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Carbamazepine-OCP Interaction — RACP Adult Medicine MCQ

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ModerateNeurologyCarbamazepine-OCP InteractionRACP Adult Medicine

A 55-year-old woman on carbamazepine for trigeminal neuralgia is commenced on the oral contraceptive pill. Despite compliance, she experiences breakthrough bleeding and becomes pregnant. What is the most likely drug interaction mechanism?

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Correct answer: B — CYP3A4 induction increasing warfarin metabolism

Carbamazepine is a potent CYP3A4 inducer, which increases the metabolism of ethinyloestradiol and progestogens in the OCP, reducing their efficacy. This is a well-documented cause of OCP failure and unintended pregnancy. Alternative contraception (copper IUD, depot medroxyprogesterone) or higher-dose OCP regimens should be used with enzyme-inducing AEDs.

Reference: AMH – 2025 – Drug Interactions; eTG Neurology 2024