Those dangers are somewhat offset by reduced risks of cancer - of the ovaries, endometrium, and digestive system - that other studies have linked to birth control pills, according to David Hunter, an Oxford University epidemiologist who wrote a commentary on the study.
Overall, the study found that women who used birth control had a 20 percent increase in their relative risk for developing breast cancer. Dr. Marisa Weiss, an oncologist and founder of a website about breast cancer, told the New York Times that "Gynecologists just assumed that a lower dose of hormone meant a lower risk of cancer". A 20 percent increase translates into only one extra breast cancer case for almost every 8,000 women.
What really surprised the researchers was that the increased risk was not confined to women using oral contraceptive pills, but also was seen in women using implanted intrauterine devices, or IUDs, that contain the hormone progestin. Still, the data show that the search for birth control drugs that don't increase breast cancer risk must continue, he said. The risk contributed by hormonal contraception, she says, is similar to the extra breast cancer risk contributed by physical inactivity, excessive weight gain in adulthood, or drinking an average of one or more alcoholic drinks per day.
The researchers concluded that hormone users overall experienced a 20 per cent increase in the relative risk of breast cancer compared with nonusers, although the risk increased with age and varied by formulation.
LINA MORCH: We found approximately a 20 percent increased risk among women who now use some type of hormonal contraception.
Past research even suggests it protects against other types of cancer.
The study was conducted by a group of scientists from Denmark who followed 1.8 million Danish women for over 10 years.
NEIGHMOND: All of these forms of hormonal contraception increased breast cancer risk by 20 percent.
The study is not likely to impact clinical practice for mammograms or contraceptive use, but women should be aware of the new information, especially if there is a history of breast cancer in their family. But he suggested doctors take time to discuss the pros and cons of different types of contraception with their patients, and that they be frank about the potential risks, suggesting women reassess hormone use as they age. However, it was commonly thought that the newer low-dose estrogen options significantly decreased - or even eliminated - that risk. What they should know, however, is that the longer they take them, the greater the chance they will develop breast cancer.
For some perspective, about 252,710 American women were diagnosed with breast cancer in 2017, according to estimates from the National Institutes of Health; 12.4 percent of women will hear the diagnosis at some point in their lives. "But we should make an individual assessment-doctor and a woman, together-to see what is the most appropriate thing for her to use". Still, the additional risk would result in a comparatively few additional cases of breast cancer, the researchers said. And epidemiologist Hunter says there are other clear benefits of hormonal contraception. And so many calculations suggest that use of all contraceptives actually prevents more cancers than it causes.