Observational series on women using the contraceptive Mirena concurrently with anti-epileptic and other enzyme-inducing drugs.

CONTEXT Contraception for women on enzyme-inducing drugs. OBJECTIVE To gather preliminary information on the contraceptive efficacy of the hormone-releasing intrauterine system (IUS) Mirena, when used concurrently with enzyme-inducers. DESIGN Observational series. SETTING/PARTICIPANTS Mirena users on enzyme-inducers were recruited from within the Margaret Pyke Centre and via doctors from throughout the UK. Data were collected systematically on structured questionnaires with particular reference to duration of Mirena use, exposure to pregnancy risk, type of concurrent medication, and reasons for drop-out. MAIN OUTCOME MEASURE Accidental pregnancies. RESULTS To date, 56 women have provided follow-up information. Most took enzyme-inducers for epilepsy. They have accumulated 1454 months of use, of which 1075 months represent exposure to pregnancy risk. Only one apparently true Mirena failure has been documented, representing a failure rate of 1.1 per 100 woman-years (95% CI 0.03-6.25). Including a second pregnancy, probably conceived after the Mirena had been removed,would raise the failure rate to 2.2 per 100 woman-years (95% CI 0.27-8.07). Although 9/30 Mirena removals were followed by re-insertion, only the first segment of use is analysed. CONCLUSION As this is a pilot study, no firm conclusions can be drawn, but our preliminary results suggest that any increased pregnancy risk, if it exists, falls within acceptable bounds.