Any Hormonal Birth Control Can Slightly Increase Breast Cancer Risk, Study Finds

Scientists have known for decades that birth control containing both estrogen and progestin can slightly increase breast cancer risk. Now, there’s evidence that progestin-only contraceptives carry a similarly increased risk.

A new study from researchers at the University of Oxford indicates that the risk of breast cancer elevates not only for those taking the pill but for people who get intrauterine devices (IUDs) and progestin shots as well.

The study, published in the journal PLOS Medicine, found that short-term use of combined and progestin-only contraceptives increases the absolute risk for breast cancer by about 0.5% for women younger than 50 years. The risk is even lower for younger women.

Progestin is a synthetic form of the human hormone progesterone. Estrogen and progestin are used in contraceptives to thicken the cervical mucus and prevent the release of eggs from the ovaries. These hormones are also involved in the development of most breast cancers.

A landmark study from 1996 attributed combined oral contraception use to a slightly increased risk for breast cancer. In the years since, progestin-only options have entered the market and become more popular.

“We know quite a lot about the risks associated with combined oral contraceptive use, but progestagen-only contraceptives have been less well studied,” Kirstin Pirie, MSc, a researcher at the University of Oxford and lead author of the study, told Verywell in an email.

The Actual Risk Isn’t That High

In the new study, the researchers gathered data from 1996 to 2017 from a large primary care database in the United Kingdom. They compared data from 9,498 women with invasive breast cancer with a group of 18,000 women in similar demographics who weren’t diagnosed with cancer.

They found that 44% of the cancer patients were prescribed hormonal birth control an average of three years prior. Comparatively, 39% of the cancer-free women received a hormonal contraceptive. That’s a difference of 20–30% in breast cancer risk, the researchers said.

In terms of actual breast cancer cases, the uptick isn’t that drastic. For every 100,000 women between the age of 16 and 20 who take hormonal birth control, an additional eight could get breast cancer.

Women between ages 35 and 39 could see an additional 265 breast cancer cases per every 100,000 users. Breast cancer becomes more common as people age, so it is expected that there will be more excess breast cancer cases in older age groups than younger ones, Pirie said.   

Besides, young adults are more likely to take hormonal birth control than those in their 30s and 40s.

The study found “no material difference” in the risk of breast cancer between people who took combination and progestin-only pills, progestin-releasing IUDs, and progestin injections.

While the study certainly finds a link between progestin and breast cancer, there isn’t enough evidence to suggest that people should stop taking birth control, said Miraj Shah-Khan, MD, medical director of the Breast Health Program at Northwestern Medicine Palos Hospital, who did not work on the study.

“It’s really important to understand what the actual risk is and that the benefits of contraception far outweigh the risks that are that are presented in this paper as well as prior publications,” Shah-Khan told Verywell.

The Pros and Cons of Hormonal Contraception and Cancer

The women in the study used contraception for an average of five years, so the researchers were only able to draw short-term conclusions about contraceptive use. The long-term effects of progestin use are still unknown, Pirie said.

The researchers say it’s not yet clear how exactly progestin factors into increasing the risk of breast cancer. Estrogen is a known driver of breast cancer. Far less is known about how progestin and progesterone factor in.

“This study is important because, generally speaking, when we think about breast cancer, we think about estrogen as being one of the driving forces of breast cancer risk,” Shah-Khan said. “The role of progestin-only is unclear.”

Many factors contribute to the development of breast cancer, including age, genetics, and some environmental exposures. The researchers say their study was large enough to account for family history differences, but that more research is needed for patients already at high risk for breast cancer.

Women who are at the highest risk for breast cancer, such as those with a BRCA 1 or BRCA2 mutation, could actually benefit from taking an oral contraceptive, according to Shah-Khan. Hormonal contraception can reduce the risk of ovarian, endometrial, and colon cancers.

“Looking at the body of literature as a whole, I think it’s important to keep in mind that this association exists, but I don’t think that it’s strong enough to say women should not do contraception,” Shah-Khan said.