Bayer Logo Bayer Logo Bayer Logo

How a Mirena IUD Works

How does Mirena work to prevent pregnancy?

  • Mirena is an IUD that works in the uterus
  • Mirena contains 52 mg of a single hormone called levonorgestrel, a type of progestin, that is often used in birth control pills
  • Because Mirena slowly releases a continuous low dose of levonorgestrel into your uterus, only small amounts of the hormone enter your bloodstream


The hormone released into your uterus by Mirena prevents pregnancy most likely by:

  • Thickening cervical mucus, preventing sperm from entering the uterus

  • Inhibiting sperm movement so it’s more difficult to reach and fertilize an egg
  • Thinning the lining of your uterus


It is not known exactly how these actions work together to prevent pregnancy.

Close-up view of what Mirena® may look like in the body
Woman painting a wall red

For women with heavy periods, Mirena can cause your menstrual bleeding to decrease by thinning the lining of the uterus.

Mirena is FDA-approved to treat heavy menstrual bleeding for up to 5 years in women who choose an IUD for birth control.



Learn More

Common side effects of Mirena include:

  • Pain, bleeding or dizziness during and after placement. If these symptoms do not stop 30 minutes after placement, Mirena may not have been placed correctly. Your healthcare provider will examine you to see if Mirena needs to be removed or replaced.

  • Changes in bleeding. You may have bleeding and spotting between menstrual periods, especially during the first 3 to 6 months. Sometimes the bleeding is heavier than usual at first. However, the bleeding usually becomes lighter than usual and may be irregular. Call your healthcare provider if the bleeding remains heavier than usual or increases after it has been light for a while.

  • Missed menstrual periods. About 2 out of 10 women stop having periods after 1 year of Mirena use. If you have any concerns that you may be pregnant while using Mirena, do a urine pregnancy test and call your healthcare provider. If you do not have a period for 6 weeks during Mirena use, call your healthcare provider. When Mirena is removed, your menstrual periods should return.

  • Cysts on the ovary. Some women using Mirena develop a painful cyst on the ovary. These cysts usually disappear on their own in 2 to 3 months. However, cysts can cause pain and sometimes cysts will need surgery.


Other common side effects of Mirena include:

  • abdominal or pelvic pain

  • inflammation or infection of the outer part of your vagina (vulvovaginitis)

  • headache or migraine

  • vaginal discharge


These are not all of the possible side effects with Mirena. For more information, ask your healthcare provider. 

Woman standing against a wall and looking at her mobile phone

Download our brochure

Get a digital version of the Mirena Patient Brochure.

Download in English Download in Spanish
A woman standing with her hands folded and looking aside

What to expect

Curious about the Mirena placement process?

Learn More


Mirena® (levonorgestrel-releasing intrauterine system) is a hormone-releasing IUD that prevents pregnancy for up to 8 years. Mirena also treats heavy periods for up to 5 years in women who choose intrauterine contraception.



  • If you have a pelvic or genital infection, get infections easily, or have certain cancers, don't use Mirena. Less than 1% of users get a serious pelvic infection called pelvic inflammatory disease (PID).
  • If you have persistent pelvic or stomach pain, or excessive bleeding after placement, tell your healthcare provider (HCP). If Mirena comes out, call your HCP and avoid intercourse or use non-hormonal back-up birth control (such as condoms or spermicide). Mirena may go into or through the wall of the uterus and cause other problems.
  • Pregnancy while using Mirena is uncommon but can be life threatening and may result in loss of pregnancy or fertility.

  • Ovarian cysts may occur but usually disappear.
  • Bleeding and spotting may increase in the first 3 to 6 months and remain irregular. Periods over time usually become shorter, lighter, or may stop.


Mirena does not protect against HIV or STIs.


Only you and your HCP can decide if Mirena is right for you. Mirena is available by prescription only.


For important risk and use information about Mirena, please see Full Prescribing Information.