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.
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.
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.
Key medicines
Depot medroxyprogesterone acetate during enzyme inductionAdminister 150 mg intramuscularly or 104 mg subcutaneously every 13 weeks using the standard schedule; no dose increase is required for hepatic enzyme induction.
Levonorgestrel emergency contraception with enzyme inductionIf copper-IUD emergency contraception is declined or unsuitable, give levonorgestrel 3 mg orally once during current induction or within 28 days of stopping, repeating if vomiting occurs within three hours.