Mirena IUD Breast Cancer Side Effects Warrant Further Studies, Researchers Warn

Several studies published in recent years have suggested a link between the popular IUD Mirena and breast cancer, but more research is needed to enhance the relevance of these findings.

Amid growing evidence that suggests side effects of the Mirena IUD may increase the risk of breast cancer, a group of Chinese scientists are calling for more research to be conducted into the findings associated with the popular birth control implant.

The Mirena is a small, t-shaped 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.

Several studies published in recent years have indicated there may be an increased risk of breast cancer in women who use it and similar levonorgestrel-releasing IUDs, such as Liletta, Kyleena and Skyla.

The first study to suggest a link was released in 2014, when Finnish researchers published a study in Obstetrics and Gynecology, which found that women implanted with the Mirena for the purpose of treating heavy periods faced a nearly 20% increased risk of breast cancer.

In a letter to the editors (PDF) submitted to the American Journal of Obstetrics and Gynecology late last month, researchers from the Shanghai First Maternity and Infant Hospital and Tongji University called for further studies into the risks that may be associated with the popular IUD.

The researchers were responding to another recent study on IUD use in Sweden, which was published in the same journal in October 2024, which again found an association between Mirena and similar levonorgestrel-releasing IUDs and cancer.

That study, also by Chinese researchers, found that the use of the Mirena and similar drug-releasing IUDs were linked to a 13% increased risk of breast cancer. Though they also found lower rates of endometrial cancer, ovarian cancer and cervical cancer.

In the recent response letter, the other team of researchers called for more studies into how body fat percentages and occupational asbestos exposure may factor into the rates of cancer. The team also called for a closer look at differences in socioeconomic and healthcare systems that may limit findings from other regions.

“In conclusion, this study represents a groundbreaking contribution to understanding the associations between LNG-IUD use and gynecologic and breast cancers, providing valuable directions for future research,” the researchers wrote. “We sincerely appreciate the authors’ work and encourage further exploration of this important area.”

The letter indicates that additional studies and discussions could help enhance the clinical relevance of these findings about the potential Mirena IUD breast cancer side effects.