Synopsis
Prevent contraceptive failure by recognising hepatic enzyme induction, selecting unaffected methods, managing short and long courses, modifying emergency contraception and separating induction from other important interactions.
- Enzyme inducers increase hepatic metabolism of contraceptive steroids and can reduce effectiveness of combined pills, patch and ring, progestogen-only pills, etonogestrel implant and oral emergency contraception.
- Copper IUD, levonorgestrel IUD and depot medroxyprogesterone acetate are not reduced by hepatic enzyme induction and are the preferred reliable options when acceptable.
- Important inducers include carbamazepine, phenytoin, phenobarbital, primidone, oxcarbazepine, rifampicin, rifabutin and St John’s wort; treat topiramate as a potential inducer regardless of dose under current CoSRH caution.
- Interaction precautions continue throughout the inducer course and for 28 days after it stops because enzyme activity does not normalise immediately.
- For short courses under two months, temporary condoms during treatment and for 28 days after may be considered with some established methods, but potent rifampicin or rifabutin needs an unaffected strategy.
- For long-term induction, switch to copper IUD, LNG-IUD or DMPA rather than relying on repeated emergency contraception or unproven dose increases.
- After unprotected intercourse with current or recent induction, offer copper-IUD EC; if unsuitable or declined, ulipristal is not recommended and levonorgestrel 3 mg may be considered with uncertain effectiveness.
- Lamotrigine is not an enzyme inducer: CHC lowers lamotrigine concentrations and the hormone-free interval raises them, so specialist management is needed for seizure control and toxicity.
Key red flags
A patient taking a teratogenic medicine such as topiramate or some antiseizure drugs without reliable unaffected contraception needs urgent specialist coordination before exposure or pregnancy occurs.
Seizure recurrence or toxicity after starting, stopping or interrupting CHC in a patient taking lamotrigine may reflect a clinically important bidirectional interaction requiring specialist review.
Pain, bleeding, collapse or a positive pregnancy test after an interaction requires urgent ectopic assessment when symptomatic and medicine-specific pregnancy advice.
Stopping an essential antiseizure, antituberculous or antiretroviral medicine to preserve contraception is unsafe; change the contraceptive plan with the relevant specialist.
Investigation priorities
Identify the inducer, dose, indication, duration and start or stop date before contraceptive failure occurs.
Management branches
A patient will use an enzyme-inducing medicine for a prolonged or uncertain duration.
- Confirm the exact inducer, teratogenic risk, expected duration, current contraception, recent intercourse and medical eligibility.
- Offer copper IUD, LNG-IUD or standard-schedule DMPA as reliable methods unaffected by induction, comparing bleeding, procedure, bone and fertility-return effects.
- Bridge safely until the new method is effective and continue inducer-related precautions for 28 days after the final dose if a vulnerable method is resumed.