Hormonal contraception, even formulations with low doses of estrogen, are associated with an increased risk of developing breast cancer—although the increase in absolute risk is small, according to a study in the New England Journal of Medicine.
According to the Associated Press, about 140 million women worldwide use some form of hormonal contraception, including about 16 million women in the United States.
Prior research has established a link between older forms of hormonal birth control, which included relatively high doses of estrogen, and breast cancer, the New York Times reports. However, according to the Times, experts had hoped that newer iterations of hormonal birth control, which rely on lower doses of estrogen, would not post similar risks.
For the latest study, funded by the Novo Nordisk Foundation, researchers looked at the health records of 1.8 million women between the ages of 15 and 49 who lived in Denmark over the course of 11 years. The researchers compared the breast cancer risk between women who had used some form of hormonal birth control—typically a combination of estrogen and progestin administered in various ways, including oral contraception, patches, vaginal rings, and injections—and women who used non-hormonal contraception, such as a condom, diaphragm, or copper intrauterine device (IUD).
The researchers excluded from the study women who had venous thromboembolism, a history of cancer (aside from non-melanoma skin cancer), or a history of treatment for infertility. The researchers adjusted for variables such as education, childbirth, and a family history of breast cancer, but acknowledged that they were unable to adjust for other cancer risk factors such as alcohol use or low physical activity, nor were they able to adjust for protective factors such as breast-feeding.
The researchers found that women taking hormonal contraception had a 20% increased risk of developing breast cancer, when compared with women who had never taken hormonal contraception. In fact, according to the researchers, the increase was comparable to the heightened risk reported in earlier research on older formulations of hormonal contraception. The increased risk was tied not only to oral contraception but also to other forms of hormonal contraception, the researchers said, with women who used IUDs that contained only progestin experiencing a 21% greater risk of cancer compared with nonusers.
However, the risk increased varied based on how long women had been using hormonal contraception and their age. Those who used hormonal contraceptives for less than a year saw a 9% increased risk, while those who used the contraception for more than 10 years saw a 38% increased risk. The researchers noted that while the risk for women who used hormonal contraception for only a brief time quickly declined, it remained heightened among women who used hormonal contraception for more than five years, even after discontinuation.
But the researchers added that the overall increased risk was small, accounting for about one extra case of breast cancer per every 7,700 women using hormonal contraception each year.
Lina Mørch, a senior researcher at the Copenhagen University Hospital and author on the study, said the findings were unexpected. "We did actually expect we would find a smaller increase in risk because today we have lower doses of estrogen in the hormone contraceptives, so it was surprising that we found this association," she said.
Mørch added that since the study showed the risk increased with length of use, it suggests the link between hormonal birth control and breast cancer is causal. "It is a very clear picture for us, very convincing," she said.
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Separately, Øjvind Lidegaard, senior author of the study, said of the study, "Nothing is risk-free, and hormonal contraceptives are not an exception to that rule." He advised providers to outline the risks and benefits of birth control with their patients and suggested that older women reconsider their use of hormonal contraception as they age.
In an accompanying editorial, David Hunter of the University of Oxford said the study is the first "with substantial data to show" that contemporary formulations of hormonal birth control still pose an increased risk of breast cancer.
However, he cautioned that the risks should be weighed alongside the benefits. "There's very good evidence that oral contraceptives reduce the risk of ovarian cancer. They reduce the risk of endometrial cancer," he said. "And there's a strong suggestion they also reduce the risk of colorectal cancer. So, many calculations suggest that the use of oral contraceptives actually prevents more cancers than it causes."
Experts reaffirm safety, efficacy of hormonal contraception
Officials with the American College of Obstetricians and Gynecologists (ACOG) said they would closely assess the study's findings, but reiterated that hormonal contraceptives, for many women, remain "among the most safe, effective, and accessible options available."
Separately, Mia Gaudet, an epidemiologist with the American Cancer Society, pointed out that "a 20 percent increase of a very small number is still a very small number." And JoAnn Manson, the chief of preventative medicine at Brigham and Women's Hospital, said while women in their 40s may want to consider alternatives, "hormonal contraception should still be perceived as a safe and effective option for family planning."
Roshni Rao, a breast surgeon at NewYork-Presbyterian/Columbia University Medical Center, added, "Oral contraceptives are like any other medication," Rao said. "There are risks and there are benefits. If you have a reason to be taking them, it's perfectly reasonable to do so" (Johnson, AP/Sacramento Bee, 12/6; Neighmond, "Shots," NPR, 12/6; Emery, Reuters, 12/6; Monaco, MedPage Today, 12/6; Rabin, New York Times, 12/6).
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