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Low-dose birth control pills and other hormonal contraceptives linked to increased risk of breast cancer

REUTERS/Eric Gaillard
The data revealed that the relative risk of developing breast cancer was 20 percent higher among women who used hormonal contraceptives than among women who had never used that form of birth control.

Women who use birth control pills or other forms of contraception that release hormones are at an increased risk of developing breast cancer, according to a Danish study published Wednesday in the New England Journal of Medicine (NEJM).

The longer hormonal contraceptives are used, the greater the risk, although the study also found that the increased risk appears to disappear five years after women stop taking such forms of birth control.

The absolute increase in risk was relatively modest — about one extra case of breast cancer among every 7,690 women — but it’s still significant, given that 140 million women worldwide, including about 16 million in the United States, use pills, patches, rings, intrauterine devices (IUDs) and implants containing hormones for birth control.

Needless to say, the study’s findings are disappointing. It had been hoped that newer, lower-dose formulations in hormonal contraceptives would be safer than the older ones. This is the first major study to look at that issue. 

More than a decade of data

For the study, researchers analyzed data collected from about 1.8 million Danish women, aged 15 to 49, over an average of about 11 years. During that period, 11,517 of the women were diagnosed with invasive breast cancer.

The data revealed that the relative risk of developing breast cancer was 20 percent higher among women who used hormonal contraceptives than among women who had never used that form of birth control.

The risk rose with longer use. Women who had used such birth control for less than a year had a 9 percent increased risk of developing breast cancer during the study, while those who had used it for more than 10 years had a 38 percent increased risk.

The risk remained elevated until about five years after the women discontinued using the contraceptives.

What do those findings mean in terms of the absolute risk to individual women? The researchers determined that for every 100,000 users of hormonal contraceptives, 13 additional breast cancers would be diagnosed each year.

The Centers for Disease Control and Prevention estimates that 252,710 women will be diagnosed with invasive breast cancer in the United States in 2017.

Balancing risks and benefits

This study was observational. That means its results show only a correlation, not a causal relationship, between the use of hormonal contraceptives and breast cancer. Although the researchers adjusted their findings to account for several other factors linked to an increased risk of the disease, including obesity, smoking status, history of pregnancies, and family history of breast or ovarian cancer, additional aspects of the women’s lives — ones not accounted for in the analysis — might also explain the study’s results.

In a commentary that accompanies the study, David Hunter, a professor of epidemiology and medicine at the University of Oxford, stresses that the breast-cancer risks identified in the current study need to be balanced with the health benefits associated with hormonal contraceptives. Such benefits include not only preventing an unwanted pregnancy, but also “substantial reductions in the risks of ovarian, endometrial and colorectal cancers in later life,” he writes.

Yet Hunter also urges the scientific community to recharge its search “for an oral contraceptive that does not elevate the risk of breast cancer.” 

“In the 1970s and 1990s, there was some optimism regarding the development of a formulation that would reduce a woman’s risk of breast cancer,” he writes,“but research into this possibility appears to have stalled.”

FMI: You can read both the study and the commentary on NEJM’s website. According to the study’s disclosure statement, the study was funded by the Novo Nordisk Foundation, which says its objective is to “provide a stable basis for the commercial and research activities of Novo Nordisk and [to] support scientific, humanitarian and social purposes.” Novo Nordisk is a pharmaceutical company whose products include hormonal drugs aimed at treating menopausal symptoms. The study’s disclosure statement also notes that two of the current study’s authors joined the foundation after the paper was published.

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