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It’s been long known that the most popular form of hormonal birth control — which contains both estrogen and a synthetic version of the reproductive hormone progesterone called progestin — is linked to a slight increase in breast cancer risk. This type of birth control is available in pill, patch or ring form.

Less is known about the breast cancer risk of progestin-only birth control. A new study completed in the U.K. and published in PLOS Medicine compares the breast cancer risk of different types of hormonal birth control for the first time. Researchers found no difference in risk between women using progestin-only birth control and those using combined progestin and estrogen methods.

Dr. Shelley Tworoger, Associate Center Director for Population Science

Dr. Shelley Tworoger, Associate Center Director for Population Science

“There’s a really limited understanding of the mechanisms for how oral contraceptives may be influencing breast cancer risk,” said Dr. Shelley Tworoger, associate center director for Population Science at Moffitt Cancer Center. “The findings could suggest that the progestins are the primary source of what’s going on to increase risk of breast cancer.”

For the study, researchers analyzed records from 10,000 female British breast cancer patients under 50 and compared the group to similar-aged women who did not have breast cancer. They found current or recent use of progestin-only birth control was associated with an increased breast cancer risk of about 20% to 30%, which is the same risk associated with combined hormonal contraceptives.

However, this extra risk has a fairly small impact because the overall risk of breast cancer in most young women is low. The average woman has a 1-in-8 chance of developing breast cancer in her lifetime, and the study suggests extra risk for those using hormonal contraceptives is about 0.2%.

Previous studies have also shown that the increased risk begins to decrease after a woman stops using any hormonal contraceptive. After 10 to 15 years of stopping use, a woman’s breast cancer risk returns to that of a woman who has never used hormonal birth control.  

Tworoger says while this is an important study, there is no need for women who are currently using any hormonal contraceptive to panic. “At this point, oral contraceptives or other forms of hormonal contraceptives provide a number of health benefits to women, including reduced risk of other types of cancer like ovarian cancer, and an improvement in health outcomes for premenstrual syndrome and endometriosis.”

According to the Centers for Disease Control and Prevention, as of 2019, 14% of women in the U.S. ages 15 to 49 took birth control pills and around 10% have intrauterine devices or birth control implants.