Phenytoin OCP Interaction — ORE Part 1 MCQ
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Correct answer: D — Reduced contraceptive efficacy through increased steroid metabolism
Explanation lettering: D = shown as A · A = shown as D
A is correct. Phenytoin induces hepatic drug-metabolising enzymes, including CYP3A4, increasing metabolism of contraceptive steroids. This can lower hormone exposure and reduce the effectiveness of a combined oral contraceptive, with breakthrough bleeding and unintended pregnancy possible. B is wrong because this is a metabolic, not an absorption, interaction. C reverses the effect: phenytoin is an inducer rather than an inhibitor. D is also mechanistically wrong; enzyme induction does not imply increased phenytoin concentration. E overstates the position: the combination is not absolutely contraindicated, but long-term phenytoin users should receive advice about effective contraception not affected by enzyme induction.
Reference: Electronic Medicines Compendium. Phenytoin Sodium Flynn Hard Capsules 100 mg: Summary of Product Characteristics, sections 4.4 and 4.6. https://www.medicines.org.uk/emc/product/7558/smpc