What we know about hormonal birth control and breast cancer risk

New research from the University of Oxford in the United Kingdom found that while all hormone contraceptives carry a small “excess” risk of breast cancer, the overall risk remains low. 

The study was published March 21 in the journal PLOS Medicine and filled in research gaps on links between breast cancer and progestagen-only contraceptives. Progestagen-only contraceptives include birth control implants, intrauterine devices (IUD), contraceptive injections, and the minipill. Other kinds of hormonal birth control contain a combination of estrogen and progestagen and include the traditional birth control pill and patch. 

[Related: Where is all the hormone-free birth control?]

Importantly, the new study does not say that hormonal contraceptives cause breast cancer. It only investigated any potential links to the disease that affects about 264,000 women and 2,400  men every year. 

“Given that the underlying risk of breast cancer increases with advancing age, the absolute excess risk associated with use of either type of oral contraceptive will be smaller in women who use it at younger rather than at older ages,” the authors wrote in a statement.“These excess risks must, however, be viewed in the context of the well-established benefits of contraceptive use in women’s reproductive years.”

The study included data on roughly 10,000 women in the UK under age 50 who were diagnosed with invasive breast cancer between 1996 and 2017. They also looked at more than 18,000 subjects who did not have breast cancer. 

They found a relative increase of 20 to 30 percent in breast cancer risk with combined birth control (which also contain estrogen in addition to progestagen) and progestagen-only contraceptives. However, after five years of use, the 15-year absolute excess incidence of breast cancer was only 265 cases per 100,000 users at most. Earlier studies show that this excess risk disappears entirely about 10 years after stopping hormonal birth control. 

“These findings suggest that current or recent use of all types of progestagen-only contraceptives is associated with a slight increase in breast cancer risk, similar to that associated with use of combined oral contraceptives,” said co-author and cancer epidemiologist Kirstin Pirie, in a statement. 

The overall risk of breast cancer in hormonal contraceptive uses is low, particularly for younger users. Additionally, the team pointed to a lack of both a complete prescription history and family breast cancer history of the women as some of the limitations in this study.

[Related: Over-the-counter birth control pills could change reproductive care in the US.]

Progestogens, or progestin, are drugs that mimic a natural hormone called progesterone, which is crucial for both menstruation and pregnancy. Progestagen-only birth control options do not contain estrogen the way that combined hormone birth control pills do. They prevent pregnancy by thickening mucus in the cervix, which stops sperm from reaching an egg. It can also completely stop ovulation in some cases.  

According to the Centers for Disease Control and Prevention (CDC), the combined hormone birth control pill is the most popular form of hormonal contraception. More people have been choosing to use the IUD or an implant over the pill for at least a decade.

Claire Knight of Cancer Research UK, who funded this study, told The Guardian that this should not discourage women from taking birth control pills. “Women who are most likely to be using contraception are under the age of 50, where the risk of breast cancer is even lower,” Knight said. “For anyone looking to lower their cancer risk, not smoking, eating a healthy balanced diet, drinking less alcohol, and keeping a healthy weight will have the most impact.”