Women seeking emergency contraception who have used medicines or herbal remedies that induce the CYP3A4 liver enzyme within the last 4 weeks should preferably use a non-hormonal emergency contraceptive, ie a copper IUD.
In women who are unable or unwilling to use a copper IUD, the usual dose of levonorgestrel should be doubled from 1.5mg to 3mg to compensate for the reduced plasma levonorgestrel levels caused by the enzyme inducer.
No increased risk of side-effects is expected from the increased dose of levonorgestrel in these circumstances.
Reduced contraceptive efficacy
Medicines that induce CYP3A4 increase the metabolism of levonorgestrel, which reduces the drug's levels in the blood and may compromise its contraceptive efficacy.
Elevated levels of CYP3A4 enzymes can persist for up to 4 weeks after cessation of an enzyme-inducing medicine.
CYP3A4 enzyme inducers include:
- Anticonvulsants (eg, barbiturates, primidone, phenytoin, carbamazepine)
- Antitubercular drugs (eg, rifampicin, rifabutin)
- Antiretrovirals (eg, ritonavir, efavirenz)
- Antifungals (eg, griseofulvin)
- St John's wort
For women taking these medicines, healthcare professionals should provide advice on highly effective ongoing contraception that is not affected by hepatic enzyme-inducing drugs ( is available from the Faculty of Sexual and Reproductive Health). This is particularly important as some of the medicines have been associated with an increased risk of birth defects when taken during pregnancy.
Women should also be advised to have a pregnancy test to exclude pregnancy after taking levonorgestrel-containing emergency contraception, and to seek prompt medical advice if they do become pregnant.
The MHRA's updated advice is in line with the above-mentioned guidance from the Faculty of Sexual and Reproductive Health, and applies to both prescription and non-prescription supply of levonogestrel-containing emergency contraception.