Mirena is a small, T-shaped birth control device that a doctor places inside your uterus. Made by Bayer, it releases a low dose of a synthetic hormone called levonorgestrel to prevent pregnancy for up to eight years. It’s one of the most effective forms of contraception available, and it doubles as a treatment for heavy periods.
How Mirena Works
The device measures just 32mm by 32mm, roughly the size of a quarter, and sits inside the uterine cavity. It contains a small reservoir of 52 mg of levonorgestrel, a synthetic version of the hormone progesterone. When first placed, it releases about 20 micrograms of this hormone per day. That amount gradually decreases, dropping to roughly half by the five-year mark.
This steady, low dose of hormone works in several ways at once. It thickens the mucus at the opening of the cervix, making it difficult for sperm to reach an egg. It also thins the uterine lining, which makes the environment less hospitable for a fertilized egg. Some people ovulate less frequently while using Mirena, though it doesn’t suppress ovulation as reliably as birth control pills do. The combination of all these effects is what makes it so effective.
How Effective It Is
Mirena is one of the most reliable contraceptives on the market. In a large extension trial, the failure rate over three years (years six through eight of use) was just 0.68%. The Pearl Index, a standard measure of contraceptive effectiveness, was 0.28 for that period. To put that in practical terms: fewer than 1 in 100 people using Mirena for three years will become pregnant. Because the device works on its own once placed, there’s virtually no difference between “perfect use” and “typical use,” unlike pills or condoms where human error plays a bigger role.
FDA-Approved Uses
Mirena carries two separate approvals from the FDA. The first is for pregnancy prevention, where it’s approved for up to eight years of continuous use. The second is for treating heavy menstrual bleeding, approved for up to five years in people who also want intrauterine contraception.
The effect on periods can be dramatic. In clinical studies, Mirena reduced menstrual blood loss by about 80% within four months, 95% by one year, and caused periods to stop entirely (amenorrhea) by two years for many users. About 20% of people using Mirena stop having periods after one year. For people with debilitating heavy periods, this can be life-changing and may help avoid surgical options like endometrial ablation or hysterectomy.
What Insertion Feels Like
The placement procedure takes only a few minutes and is done in a regular office visit, not an operating room. You’ll lie in the same position as a pelvic exam. Your provider inserts a speculum, cleans the cervix, and measures the depth of your uterus with a thin instrument called a sound. The uterine cavity typically measures between 6 and 9 cm. If it’s shorter than 6 cm, the device can’t be placed safely. The Mirena is then loaded into a thin inserter tube and guided through the cervix into position. Once placed, the strings are trimmed to about 3 to 4 cm so they extend just past the cervix.
Most people feel cramping during and immediately after insertion, ranging from mild to intense. Taking an anti-inflammatory pain reliever beforehand can help, and some providers offer a numbing injection near the cervix (a local anesthetic block) to reduce discomfort during the process. A follow-up visit is typically scheduled four to six weeks later to confirm the device is still in the right position.
Common Side Effects
The most frequently reported side effects include headaches, acne, breast tenderness, mood changes, and cramping or pelvic pain. Irregular bleeding or spotting is very common in the first three to six months. This is the side effect that frustrates most new users, but it typically improves significantly after the first three months and continues to lessen over time. For many people, periods eventually become very light or stop altogether.
Rare but Serious Complications
Two complications worth knowing about are perforation and expulsion. Perforation is when the device pushes through the wall of the uterus. This is uncommon: the five-year cumulative incidence is about 0.6% overall. The risk is lowest for people who aren’t recently postpartum (about 0.3% over five years) and highest when the device is placed between four days and six weeks after delivery (about 2%).
Expulsion means the device partially or fully slips out of place. This happens more often than perforation. The overall five-year expulsion rate is roughly 4.6%. Timing matters here too. People who get Mirena placed in the first three days after giving birth have the highest five-year expulsion rate, around 10.7%, while those who wait more than six weeks postpartum actually have a lower rate (about 3.2%) than those who weren’t recently pregnant. If the device shifts or comes out, it no longer provides reliable protection.
How Mirena Compares to Other Hormonal IUDs
Mirena isn’t the only hormonal IUD available. Liletta is the most similar: it’s the same size (32mm by 32mm) and contains the same 52 mg of levonorgestrel, releasing a comparable daily dose. The two are often considered interchangeable, though their approved durations differ slightly.
Kyleena and Skyla are physically smaller (28mm by 30mm), which can make insertion easier for people with a smaller uterus or who haven’t been pregnant. The tradeoff is a lower hormone dose. Kyleena contains 19.5 mg of levonorgestrel and releases about 17.5 micrograms per day initially. Skyla contains just 13.5 mg, releasing about 14 micrograms per day, and is approved for only three years. These lower-dose options are less likely to stop periods entirely, which is a positive for some people and a drawback for others who are specifically seeking lighter bleeding.
Fertility After Removal
Mirena’s effects reverse quickly once the device is removed. In studies of people who had their IUD taken out to try to conceive, 94.3% became pregnant. More than half of those pregnancies (55.9%) happened within the first three months after removal. This rapid return to fertility is one of Mirena’s advantages over long-acting injections, which can delay the return of ovulation for months. Removal itself is a brief office procedure, generally quicker and less uncomfortable than insertion.