Emergency contraception during carbamazepine therapy in an adolescent — DFSRH MCQ

Correct answer: B — Levonorgestrel 3 mg as a single dose

Explanation lettering: E = shown as C · C = shown as E

Carbamazepine induces hepatic drug-metabolising enzymes, principally CYP3A4, and can reduce exposure to both levonorgestrel and ulipristal acetate. A copper IUD is unaffected by enzyme induction, is the most effective emergency contraceptive method and should be offered to adolescents; however, this patient has declined it. Because the episode occurred within 72 hours, the appropriate oral alternative is levonorgestrel 3 mg taken as a single dose. She should be advised that the effectiveness of this adjusted regimen during enzyme-inducer use remains uncertain. A provides the same total levonorgestrel dose but uses an unsupported divided regimen; the two 1.5 mg tablets should be taken together. C is the usual levonorgestrel dose for someone not exposed to an enzyme inducer and may provide inadequate exposure during carbamazepine treatment. D would ordinarily be a valid and often preferred oral option within 120 hours, but carbamazepine can substantially reduce ulipristal exposure; it would become appropriate if she had not used an enzyme inducer during the preceding 28 days and ulipristal were otherwise suitable. E is incorrect because doubling the ulipristal dose is not recommended: there are no efficacy data supporting this strategy. Ongoing contraception compatible with enzyme-inducing medication, prevention of further unprotected intercourse and follow-up pregnancy testing should also be addressed.

Reference: FSRH Guideline: Emergency Contraception (March 2017, amended April 2026) — https://www.fsrh.org/Common/Uploaded%20files/documents/fsrh-guideline-emergency-contraception03dec2020-amendedjuly2023-11jul.pdf LoviOne 1.5 mg Tablet — Summary of Product Characteristics (Text revised 15 April 2026; emc updated 15 June 2026) — https://www.medicines.org.uk/emc/product/15062/smpc