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.