The latest MHRA Drug Safety Update details revised guidance for healthcare professionals on the use of levonorgestrel emergency contraception in patients treated with enzyme inducing drugs.
This updated guidance advises that women who are currently using or have used drugs that increase CYP3A4 enzyme activity within the previous 4 weeks should be advised that the most effective form of EHC is a copper IUD. Where this is not an option women should be advised that they require double the standard dose of Levonorgestrel (i.e. 2x1.5mg tablets).
Levonorgestrel containing emergency contraception is available to prevent unwanted pregnancy when taken within 72 hours of unprotected intercourse or failure of contraception. The sooner the levonorgestrel is taken the more effective the treatment will be.
Levonorgestrel is metabolised by the CYP3A4 enzymes, concomitant administration of drugs that induce these enzymes will increase the metabolism of the levonorgestrel decreasing efficacy. Elevated levels of these enzymes can persist for up to 4 weeks after cessation of treatment. Consequently, women choosing to use levonorgestrel as emergency contraception who are either currently using or have used within the last 4 weeks an enzyme inducing drug are advised to take double the standard dose of levonorgestrel.
Examples of drugs expected to induce CYP3A4 enzymes and reduce plasma levels of levonorgestrel include:
- Epilepsy e.g. barbiturates, primidone, phenytoin and carbamazepine
- Tuberculosis e.g. rifampicin, rifabutin
- HIV e.g. ritonavir, efavirenz
- Fungal infections e.g. griseofulvin
- Herbal remedies e.g. St John’s Wort (hypericum perforatum)
This is not an exhaustive list further examples can be obtained from the BNF.
Numark members with specific queries can contact Numark Information Services on 0800 783 5709 option 2.
Pharmacists are reminded that the licensed dose for OTC sale of levonorgestrel remains as a single 1.5mg dose, consequently women requesting levonorgestrel who are currently taking or have taken (within the past 4 weeks) an enzyme inducing drug should be referred to their GP or sexual health clinic for treatment.
MHRA have produced an information sheet to support this updated guidance that pharmacists can give to their patients, this leaflet is available from GOV website.