Why can an epilepsy medicine make the pill fail?
Taken perfectly, the pill can still fail — when the liver clears it too fast.
Select any card for the reasoning, what to watch for and an exam tip.
The puzzle
A 24-year-old woman takes a combined pill every day and never misses one. She starts carbamazepine for epilepsy — and a few months later the pregnancy test is positive.
The answer, step by step
Who is most at risk
- Antiepileptics: carbamazepine, oxcarbazepine, eslicarbazepine, phenytoin, phenobarbital, primidone, topiramate.
- Rifampicin and rifabutin; efavirenz, nevirapine and ritonavir.
- St John’s wort — bought over the counter, so ask about it.
What to do
Key takeaway
Enzyme inducers such as carbamazepine speed up the liver’s clearance of contraceptive hormones — so switch to a method they can’t touch, for the course and 4 weeks after.