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Phenytoin and oral contraceptive — GPhC CRA MCQ

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ModerateDrug InteractionsPhenytoin and oral contraceptiveGPhC CRA

A patient taking phenytoin asks for advice after starting a combined oral contraceptive. They have had breakthrough bleeding. What is the most important interaction to flag?

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Correct answer: B — Reduced contraceptive effectiveness from enzyme induction

Phenytoin induces hepatic enzymes and can reduce hormonal contraceptive effectiveness. Specialist contraceptive advice is needed. In this scenario, the correct option is reduced contraceptive effectiveness from enzyme induction. The other options are plausible but do not address the key clinical, legal or safety issue as directly. This aligns with UK pharmacy practice guidance cited in the reference field.

Reference: BNF; FSRH