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:
• Thickening cervical mucus to prevent sperm from entering your uterus
• Inhibiting sperm from reaching or fertilizing your egg
• Thinning the lining of your uterus
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 releases small amounts of levonorgestrel, a progestin hormone found in many birth control pills, locally into your uterus at a slow rate. Because of this, only small amounts of the hormone enter your blood. Mirena does not contain estrogen.
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.