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Lamotrigine–combined hormonal contraception interaction in epilepsy — DFSRH MCQ

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HardNeurologyLamotrigine–combined hormonal contraception interaction in epilepsyDFSRH

A 32-year-old woman with focal epilepsy requests a change of contraception. Nine months ago, she started a combined oral contraceptive containing ethinylestradiol 30 micrograms and levonorgestrel 150 micrograms in a 21/7 regimen. Before this, her seizures were controlled with lamotrigine 200 mg twice daily. After starting the pill, she had two focal impaired-awareness seizures with a substantially reduced lamotrigine concentration; her neurologist progressively increased lamotrigine to 350 mg twice daily, restoring seizure control. During each subsequent hormone-free interval, she has developed transient diplopia, dizziness and gait ataxia. Paired measurements show that her lamotrigine concentration near the end of the hormone-free interval is approximately twice that during active-tablet use. She is currently on day 16 of active tablets, has used them correctly and had condomless intercourse 2 days ago. Pregnancy testing is negative. She wants highly effective contraception that is likely to reduce menstrual bleeding and accepts a 52 mg levonorgestrel intrauterine device. There are no contraindications to insertion. Which is the most appropriate management plan?

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

Reveal the answer and explanation

Correct answer: C — Insert the levonorgestrel intrauterine device now, stop the combined pill, and coordinate prompt lamotrigine dose reduction and monitoring with her neurologist

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

The correct plan is to insert the 52 mg levonorgestrel intrauterine device now and stop the combined pill, while coordinating reduction and monitoring of lamotrigine. She is in week 3 of correctly used combined hormonal contraception, so immediate switching does not require additional contraceptive precautions; the recent intercourse does not require delayed insertion. Levonorgestrel intrauterine contraception is highly effective, may reduce bleeding and is not expected to lose contraceptive effectiveness during lamotrigine use. Ethinylestradiol induces lamotrigine glucuronidation, approximately doubling its clearance. This explains the breakthrough seizures after pill initiation and the required dose escalation. Lamotrigine clearance falls when oestrogen is withdrawn, causing the observed hormone-free-interval toxicity. Abruptly stopping the pill while maintaining 350 mg twice daily could therefore produce sustained toxicity, so specialist-guided dose reduction and clinical or concentration monitoring are required. A applies an unnecessary 7-day overlap and fails to address the predictable rise in lamotrigine after pill cessation. B incorrectly treats intercourse during correctly used week-3 pills as unprotected and creates an avoidable contraceptive gap. D is less suitable because lamotrigine may modestly reduce systemic progestogen exposure; FSRH advises additional condoms with the implant because its effectiveness might be reduced. E may reduce cyclical concentration changes, but continued oestrogen maintains increased lamotrigine clearance and is inferior to an acceptable method unaffected by the interaction.

Reference: FSRH Clinical Guidance: Drug Interactions with Hormonal Contraception (May 2022) — https://www.fsrh.org/Common/Uploaded%20files/documents/drug-interactions-with-hormonal-contraception-5may2022.pdf FSRH Guideline: Combined Hormonal Contraception (January 2019, amended October 2023) — https://www.fsrh.org/Common/Uploaded%20files/documents/fsrh-guideline-combined-hormonal-contraception-october-2023.pdf Lamotrigine 50 mg tablets: Summary of Product Characteristics (2026) — https://www.medicines.org.uk/emc/product/4737/smpc