Specifically, the Mirena IUD is a small, “T-shaped” device that is inserted into the uterus by a physician or health care provider and is intended to prevent pregnancy. The hormonal device is coated with a certain level of “levonorgestral,” a type of progestogen, which effectively regulates the release of more levonorgestral. This hormonal regulation is designed to prevent the possibility of pregnancy.
Since its introduction, Bayer Healthcare has faced pressure from lawsuits and warnings from the FDA about its continual failure to warn about the side effects caused by the Mirena IUD.

Source: good-legal-advice.com