However, due to limitations of the available data a precise estimate of the pregnancy rate is not possible. In clinical trials, side effects of Mirena were most common during the first months after insertion and subsided during prolonged use. In addition to preventing pregnancy, Mirena use may result in a change in bleeding patterns. There may be an increase in intermenstrual bleeding in the first three to six months following insertion. After this, a woman's period may become shorter and lighter.
Some women experience an absence of menstrual bleeding after one year.
Please contact a health care provider for a complete list of possible Mirena side effects or for more information. The local mechanism by which continuously released levonorgestrel enhances contraceptive effectiveness of the IUD has not been determined. For additional information on Mirena, including Patient Information and the Physician Insert, please visit Berlex Laboratories .