Mirena is more than 99% effective at preventing pregnancy for as long as you want for up to 5 years.
Mirena prevents pregnancy, most likely in several ways:
• Thickens cervical mucus to prevent sperm from entering your uterus
• Inhibits sperm from reaching or fertilizing your egg
• Thins the lining of your uterus
Mirena may stop the release of your egg from your ovary, but this is not the way it works in most cases. While there’s no single explanation for how Mirena works, most likely, the above actions work together to prevent pregnancy for up to 5 years.
Mirena does not protect against HIV or STDs.
As soon as Mirena is placed in your uterus by your healthcare provider, it starts releasing small amounts of the hormone levonorgestrel into your uterus to provide continuous birth control for as long as you want, for up to 5 years.
Mirena and hormones
Mirena is estrogen-free. It releases small amounts of levonorgestrel, a progestin hormone found in many birth control pills, locally into your uterus at a slow and steady rate. Only small amounts of hormone enter your blood.
Mirena may also help treat heavy periods in women who choose intrauterine contraception. The hormone in Mirena reduces the monthly thickening of the uterine lining.