Studies of older birth control pills have shown "a net cancer benefit" because of lowered risk of cancer of the colon, uterus and ovaries despite a raised breast cancer risk, said Mia Gaudet, a breast cancer epidemiologist at the American Cancer Society.
Women taking modern formulations of the pill have a 20 percent increased risk of breast cancer compared with those who've never been on hormonal contraception, the study of nearly 2 million Danish women found. "There was some suggestion in the paper that women might want to consider changing their contraceptive method when they get into their 40s, when their overall risk of breast cancer does start to increase".
What those numbers mean in terms of actual women getting breast cancer who otherwise may not have is a bit less striking: there was about one extra breast cancer case diagnosed for every 7690 women who used hormonal contraception for a year.
Breast cancer is the second-biggest cancer killer of American women, after lung cancer. That is, for every 100,000 women using hormonal birth control, there are 68 cases of breast cancer annually, compared with 55 cases a year among non-users.
"No one should take (oral contraceptives) without careful thought, but the advantages in avoiding an unwanted pregnancy will usually more than outweigh the very slightly increased risk of breast cancer", said Ashley Grossman, emeritus professor of endocrinologyat Britain's University of Oxford.
The bottom line is that before starting or continuing to take hormonal contraceptives - or any medications - it's important to speak with your doctor about any potential risks and benefits, and make an informed decision from there.
Morch's team pored through years of electronic health records collected by the Danish health system, using prescription data to identify which women had taken the drugs and then track their health outcomes.
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Officials with the American College of Obstetricians and Gynecologists said that they would carefully evaluate the new findings, but emphasized that hormonal contraceptives are for many women "among the most safe, effective and accessible options available".
Modern birth control pills that are lower in estrogen have fewer side effects than past oral contraceptives.
On the other hand, among women who used hormonal contraceptives for short periods, the excess risk of breast cancer disappeared rapidly after use was stopped, the researchers said.
In fact, birth control increases breast cancer risk about as much as drinking alcohol does, said Dr. Mary Beth Terry, an epidemiologist at the Columbia University Mailman School of Public Health. Still, the additional risk would result in a comparatively few additional cases of breast cancer, the researchers said.
Today, most versions of the pill contain between 15 and 35 micrograms of estrogen, Gaudet said. Don't forget there is relative risk of death in pregnancy, too. But by the time a woman reaches 40, her probability of developing breast cancer in the next 10 years is 1.45 per cent, or 1 in 69. By contrast, there was no increased risk for breast cancer seen in women who used hormones for less than one year. "In particular the knowledge of risk with newer progestins was sparse".
The new study looked at all women in Denmark ages 15 to 49 who had not had cancer, clots in their veins, or treatment for infertility.
The risk was 9 percent higher with less than one year of use and 38 percent higher with more than 10 years of use.
"No type of hormone contraceptive is risk-free unfortunately", said lead author Lina Morch of Copenhagen University Hospital. Condoms and diaphragms do not deliver hormones.