Women's Health Matters

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Mirena IUD (intrauterine device)

Mirena is a unique type of intrauterine device (IUD) that contains low doses of progestin: the same hormone used in some birth control pills. Like other IUDs, the Mirena is a small T-shaped device placed into the uterus by your healthcare provider. Because the hormones are released directly into the uterus, only low doses are necessary to prevent pregnancy, about 1/7 of that in a birth control pill. Mirena is about 99+ per cent effective in preventing pregnancy, similar to the birth control pill. This is significantly higher than a conventional copper IUD, which is 98-99 per cent effective. Mirena does not protect you and your partner against sexually transmitted infections.

Copper IUDs can cause longer, heavier, crampier periods, so are not recommended for women with long, heavy, crampy menstrual periods. Mirena will reduce a woman's periods, and may be a useful treatment for women with small to moderate fibroids or heavy menstrual bleeding.

Using Mirena

Mirena must be inserted in a doctor's office or clinic. It is often inserted during your period or shortly after having an abortion or giving birth when your caregiver can be sure that you are not pregnant. The insertion takes about five minutes. It is inserted into the uterus so that thin strings attached to the base of the IUD hang down through the cervical opening. These strings are trimmed so they are just long enough for you to check that the IUD is in place and so that a healthcare provider can use them to remove the IUD at a later date. Most women have some cramps when the IUD is being inserted.

You may want to use another method of birth control as back-up for a month in case the IUD moves or comes out. After six weeks, return to your doctor or clinic for a check-up then continue to have your regular check-ups. The Mirena can remain in place for five years and can be easily and quickly removed by a healthcare provider.

Changes to your menstrual cycle and other side effects

Mirena will change your menstrual cycle and eventually make your periods much lighter. However, in the first three months of use, many women experience unpredictable bleeding, and about 20 per cent have prolonged periods that last more than eight days. After three months, bleeding becomes more regular and may stop altogether. In studies of Mirena, women's periods were reduced by 85 per cent after three months and 97 per cent after a year. Although bleeding is much lighter, some women who use Mirena have difficulty predicting when their period will occur.

Beyond these menstrual irregularities, Mirena has few side effects because it contains such a small amount of hormone. Some women do experience mild side effects related to the hormones such as mood changes, skin changes, water retention, breast tenderness or acne. Often these resolve after the first three months of use.


  • very effective
  • neither partner can feel an IUD during sex
  • the woman controls this method of birth control
  • can remain in place for five years


  • cost
  • does not protect against sexually transmitted infections
  • unpredictable bleeding and other side effects particularly in the first few months
  • increased risk of pelvic infection mainly associated with insertion
  • increased risk of benign ovarian cysts
  • may be difficult to insert in some women
  • rarely may puncture the wall of the uterus


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  • A publication of:
  • Women's College Hospital