Objective To gather preliminary information on the contraceptive efficacy of the hormone-releasing IUS Mirena® when used concurrently with enzyme-inducers.

Mirena® users on enzyme-inducers were recruited from within the Margaret Pyke Centre and via doctors throughout the UK. Data were collected systematically on structured questionnaires with particular reference to the duration of Mirena® use, exposure to pregnancy risk, type of concurrent medication, and reasons for drop-out.

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 right Mirena® failure has been documented, representing a failure rate of 1.1 per 100 woman-years. Including a second pregnancy, probably conceived after the Mirena® had been removed, would raise the failure rate to 2.2 per 100 woman-years. Although 9/30 Mirena® removals were followed by re-insertion, only the first segment of use is analyzed.

The study concluded that 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.

Reference: https://srh.bmj.com/content/28/2/78