Link Between Mirena IUD and Breast Cancer Side Effects Strengthened in New Study

A new study appears to supply more evidence that women may face breast cancer side effects from the Mirena IUD and other levonorgestrel-releasing birth control implants, which may not have been adequately disclosed by the drug maker.

The Mirena is a small, t-shaped intrauterine device (IUD), which is placed into the uterus to provide long-term protection against pregnancy for up to five years. It is the best selling form of IUD birth control in the United States, featuring a polyethylene frame that contains a steroid reservoir that releases levonorgestrel, which is a second generation progestin.

According to data published in a research letter in the Journal of the American Medical Association (JAMA) on October 16, researchers from Denmark indicate that the Mirena and similar levonorgestrel-releasing IUDs, including Liletta, Kyleena and Skyla, appear to increase the risk of breast cancer in women by about 30%, which supports the findings of earlier research.

Concerns about the link between Mirena IUD and breast cancer first arose in 2014, when Finnish researchers published a study in Obstetrics & Gynecology, which found that women who receive the Mirena IUD to treat heavy periods may face a greater risk of breast cancer. Those researchers saw a nearly 20% increase in the risk of breast cancer with levonorgestrel IUDs.

As a result of those findings, at least one Mirena IUD breast cancer class action lawsuit was filed by Priya Sidhu in March 2022, alleging that the manufacturer, Bayer, knew or should have known about the potential risks for years, but withheld warnings from users and the medical community.

In this new study, a research team from The Danish Cancer Institute, led by Dr. Lina Steinrud Mørch, looked at data on nearly 80,000 women between the ages of 15 and 49 who used the Mirena or similar levonorgestrel IUDs.

According to the findings, their risk of breast cancer was 40% higher than women who did not use those types of IUDs. They found that IUD recipients suffered an additional 14 cases of breast cancer per 10,000 women over five years. While there were even more cases when the IUDs were used for longer than five years, there were not enough of such users for the researchers to reach statistically significant conclusions.

The study does note that the Mirena IUD breast cancer risk was still comparatively small, and about equivalent with the risk associated with taking oral contraceptives. However, researchers warned doctors to inform patients considering an IUD about the potential breast cancer risks.

In October 2023, the Mirena IUD class action lawsuit over the breast cancer side effects was initially cleared to move forward by the U.S. District Judge presiding over the claim, after a motion to dismiss filed by Bayer was rejected. However, following further discovery proceedings the parties filed a stipulation of dismissal (PDF) in May 2024.

In prior years, Bayer has already faced thousands of Mirena IUD lawsuits over various problems with the implant, involving allegations that the drug maker failed to adequately warn about potential side effects of the IUD birth control.

Most of the cases involved women who suffered severe internal injuries after a Mirena IUD migrated out of place, resulting in perforations of the uterus and other complications. However, the drug maker paid Mirena IUD settlements to resolve those claims while hundreds of lawsuits were pending on appeal.