Mirena is a hormonal-releasing intrauterine device, marketed by Bayer, that is placed in the uterus to prevent fertilization and act as a long-term contraceptive. It is also used to ease pain and other symptoms of endometriosis.

How Mirena works

The active ingredient in Mirena is levonorgestrel, a synthetic progesterone-like hormone that binds to and activates progesterone receptors. Progesterone is a female hormone that plays an essential role in the menstrual cycle and during pregnancy.

During the phase of the reproductive cycle in which the egg cell matures, the levels of another female hormone — estrogen — gradually increase and cause the endometrium, or lining of the uterus, to thicken. After the release of the mature egg from the ovary, the level of progesterone starts to rise. This inhibits the further thickening of the endometrium, and mitigates the effects of estrogen. At the same time, progesterone induces the secretion of fluids in the endometrium to prepare the uterus for implantation, or for the fertilized egg to attach to it. If no implantation occurs, progesterone levels decrease to trigger menstrual bleeding.

Mirena continually releases levonorgestrel, which binds to progesterone receptors and partially mimics the action of progesterone. As such, levonorgestrel works to counteract the effect of estrogen on endometrial tissue in the uterus. It is thought that it similarly interferes with the growth of endometriotic lesions outside the uterus, and by doing so reduces endometriosis-associated symptoms.

With continued use of Mirena, the menstrual bleeding diminishes, and after one year of use a woman’s period might stop completely. As a consequence of lighter and shorter — or no — menstrual cycles, pain and other menstruation-associated symptoms of endometriosis should ease.

Mirena in clinical trials

In a randomized controlled trial, 82 women with endometriosis were randomized to receive either Mirena or a gonadotropin-releasing hormone (GnRH) agonist called Lupron. The Mirena coil was inserted and left in the uterus for six months for patients on this treatment; others were given a Lupron injection (3.75 mg into a muscle) every 28 days for a total of six doses.

In both groups, chronic pelvic pain decreased significantly within six months, with no differences seen among these patients. The pain decreased more rapidly in women with stage 3 or 4 endometriosis, compared to women with milder, stage 1 or 2 disease.

Menstrual bleeding was assessed with a score calculated from bleeding calendars. The Mirena group had higher bleeding scores than the GnRH agonist group at all assessed time points. In the Mirena group, 70 percent of women reported no bleeding in the sixth treatment month, compared to 98 percent in the GnRH group. There was no reported differences in quality of life between the groups. But the researchers in Brazil who evaluated this trial noted advantages to Mirena’s use, such as “the fact that it does not provoke hypoestrogenism and that it requires only one medical intervention for its introduction every 5 years.”

Additional information

Mirena should not be used in women who are or may be pregnant, or those trying to conceive.

Common side effects of Mirena are pelvic pain, acne, and headaches. Heavy and irregular bleeding mainly occurs during the first six months and improves with prolonged use.

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Sarah Neidler Editor
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Sarah Neidler Editor