skip to main content

Carbamazepine enzyme induction — MRCPsych Paper A MCQ

Instant feedback + full explanation. One question, done properly.

ModeratePsychopharmacologyCarbamazepine enzyme inductionMRCPsych Paper A

A 37-year-old woman taking carbamazepine for bipolar disorder starts a combined oral contraceptive. Despite taking the contraceptive consistently and correctly, she later has an unplanned pregnancy. Which pharmacological mechanism most likely accounts for the contraceptive failure?

Educational content. Not a substitute for clinical judgement or local policy.

Reveal the answer and explanation

Correct answer: A — Induction of hepatic CYP3A4 increasing metabolism of contraceptive steroids

Carbamazepine is a potent inducer of hepatic CYP3A4 and other drug-metabolising enzyme systems. It increases the metabolism and clearance of the oestrogen and progestogen components of combined hormonal contraception, lowering hormone exposure and increasing the risk of contraceptive failure. Therefore, A is correct. Reduced renal clearance or plasma protein displacement would tend to increase hormone exposure rather than reduce efficacy. Carbamazepine does not cause contraceptive failure through dopamine receptor blockade. CYP3A4 inhibition would reduce steroid metabolism and generally increase, rather than decrease, contraceptive hormone concentrations; these steroids do not depend on CYP3A4 activation. Women taking carbamazepine require counselling about effective contraception that is not compromised by enzyme induction.

Reference: Novartis Pharmaceuticals UK Ltd. Tegretol 200 mg Tablets: Summary of Product Characteristics, sections 4.4–4.6; text revised 20 April 2026. https://www.medicines.org.uk/emc/product/7845/smpc