The Mirena intrauterine system (IUS), the IUD with the hormone levonorgestrel, is a highly effective method of contraception currently approved for five years. Some data suggests that it probably good for six years, but a new study tells us with a good degree of confidence that the Mirena is safe and effective for seven years. Yes, seven years.
The study was funded by UNDP/ UNFPA/WHO/World Bank Special Programme of Research and published in the journal Contraception. The investigators compared the Mirena IUS head to head against the copper IUD. Almost 4,000 women participated in the study and they were randomized, meaning neither the women nor their physicians were allowed to select the IUD (this helps to remove some kinds of bias). The women were followed for pregnancies and reasons for removal.
By the end of year seven only 0.53% of women with the Mirena IUS had became pregnant versus 2.45% for the copper IUD. There were seven pregnancies among 1,884 women. While the copper IUD is an excellent form of birth control the Mirena was even more effective. In fact, it is far better than any other method of birth control. Based on this data a pregnancy with a Mirena is almost a reportable event, that’s how rare it is.
Along the way many women had their IUDs removed, typically because they desired pregnancy or had side effects. There were 717 Mirena users who completed six years of follow-up and 398 women who completed seven years with no pregnancies in year 6 or 7. With the copper IUD there were four pregnancies in total in years six and seven. There were slightly more copper IUD users in year six and seven as early removal was more common among Mirena IUD users and this was driven largely by women at the Chinese centers who don’t find amenorrhea (lack of periods), a side effect of the Mirena, to be acceptable. An interesting cultural difference as amenorrhea is more often than not valued by contraceptive users in the United States.
What also stands out from the study is the absence of ectopic pregnancies in the Mirena arm. Let me state that again because it’s really important, not one women out of the almost 2,000 who had a Mirena had an ectopic pregnancy. Three of the 33 pregnancies with the Copper were ectopic. It’s possible that there were just too few pregnancies with the Mirena to detect ectopic pregnancies, but it’s also possible that either levonorgestrel changes cervical mucus in such a way that sperm has no chance or that the local hormonal effects are as good in the fallopian tube as they are in the endometrium. It’s not really possible to say as he exact method of pregnancy prevention isn’t known. We know it’s not ovulation suppression because by year four 75% of cycles with the Mirena are ovulatory.
If a Mirena IUD is $700 for insertion (that’s just an average) in addition to the convenience of extending the lifespan for two more years the cost per year will be $100 instead of $140.
Could he Mirena be good for eight years? The manufacturer was worried that there wouldn’t be enough levonorgestrel for an adequate endometrial effect (one proposed mechanism of action), so a plan to extend the study to eight years was scrapped. Given the lack of pregnancies in year 6 and 7 with the Mirena and a cumulative pregnancy rate of 0.53% one has to wonder if the Mirena would perform better than the copper and its 2.45% failure rate at the end of year eight.
Bayer has little incentive to prove the Mirena lasts longer. They make money by selling new product. It’s unlikely they will submit to the FDA for a change in product labeling or invest in a study to test efficacy after eight years. Hopefully the WHO will invest in this because the longer a woman can use a highly effective and reversible method of contraception the better.
So good news! The Mirena is not only the most effective form of birth control and has a stunningly low ectopic pregnancy rate, but it’s also good for seven years!