How effective is mirena (levonorgestrel)?

Very effective: Mirena (levonorgestrel) is one of the most effective non-permanent methods of birth control. It is better than 99% effective.

99.5%: The Mirena (levonorgestrel) IUD is one of the BEST forms of birth control available, it lasts 5 years, is ~99.5% effective, and thins the uterine lining so that you get lighter periods. 20% of women will stop having periods after 1 year and that's perfectly healthy. Effects in the uterus and so it won't affect or be affected by other medicadtion; very safe method; biggest risk after insertion is expulsion (1-2%).