enzyme inducing drugs and emergency contraception

Last edited 08/2018 and last reviewed 05/2019

If emergency contraception is required for women and girls taking enzyme-inducing anti-epileptic drugs (AEDs), the type and dose of emergency contraception should be in line with the SPC and current edition of the BNF (1)

Other guidance states that (2)

  • women using enzyme-inducing drugs who require emergency contraceptionn should be advised of the potential interactions with oral methods and offered a copper intrauterine device (Cu-IUD). If a Cu-IUD is unacceptable or unsuitable, a double dose of levonorgestrel EC (LNG-EC) can be used
      • can use DOUBLE DOSE i.e. 3 mg (2 x 1.5 mg tablet) as a single dose within <72 hours of unprotected sexual intercourse (UPSI) if Cu-IUD is declined or unsuitable
        • effectiveness of 3 mg LNG is unknown in this situation

      • ulipristal acetate (UPA) is not advised as emergency contraception in women using enzyme-inducing drugs or who have taken them within the last 28 days

Advice for women using drugs that may reduce contraceptive efficacy (2)

    • women using drugs that affect gastric pH (e.g. antacids, H2 antagonists and proton pump inhibitors) and who require EC should be offered a Cu-IUD or LNG as the efficacy of UPA may be reduced

Drugs which induce liver enzymes include:

  • enzyme inducing drugs that may decrease contraceptive efficacy:

    • antiepileptics (e.g. carbamazepine, eslicarbazepine, fosphenytoin, oxcarbazepine, phenobarbital, phenytoin, primidone, rufinamide, topiramate)

    • antibacterials (e.g. rifabutin, rifampicin)

    • antiretrovirals (e.g. efavirenz, nevirapine). Ritonavir reduces the bioavailability of estrogen and may reduce the bioavailability of progestogens by inducing glucuronidation

    • antidepressants (e.g. St John's wort - a herbal preparation)

    • others (e.g. modafinil, bosentan, aprepitant, lumacaftor)

Women who are using enzyme inducing drugs and are in need of EC should be informed that a copper intrauterine device (Cu-IUD) is the most effective method of EC (2).


  1. NICE (April 2018). Epilepsies: diagnosis and management
  2. Faculty of Sexual & Reproductive Healthcare Clinical Guidance.Clinical Guidance: Drug Interactions with Hormonal Contraception January 2018.