Mechanism of Action
How does Mirena® work?
The contraceptive effect of Mirena® is based on a local effect of levonorgestrel
The cervical mucus is thickened making it difficult for the sperm to get through to fertilise the egg.
Normal sperm movement and function inside the uterus and ovarian tubes is inhibited.
The growth of the lining of the uterus (endometrium) is reduced, which also makes menstrual blood loss much lighter and shorter.
Due to these changes in the endometrium, many women will have spotting and irregular bleeding during the first 3-6 months following the insertion of Mirena®. Thereafter, the strong suppression of the endometrium results in shorter and lighter periods.
Understanding the bleeding pattern is an essential aspect of counselling and appropriate patient education has been shown to promote continuation
PP-MIR-IE-0070-1 | January 2026