Study: A Curious Link Between Birth Control Pills and Prostate Cancer

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Could women’s use of birth control pills increase men’s risk of prostate cancer? A new study in the medical journal BMJ Open suggests there’s a link, finding that countries where more women take oral contraception have higher rates of prostate cancer and prostate cancer deaths.

It isn’t entirely clear why the use of birth control pills would affect cancer in men, but the authors, Dr. David Margel, a urologist at Princess Margaret Hospital, and Dr. Neil Fleshner, head of urology at the University Health Network, in Toronto, suggest that it may have to do with environmental exposure: estrogen in birth control pills may be released in urine and end up in the water supply.

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Some previous research has suggested that a link between exposure to estrogenic endocrine-disrupting compounds and prostate cancer is plausible: for example, the study authors note, one study of farmers found a weak but significant link between acres sprayed with herbicides — which are known to have endocrine-disrupting properties — and a higher risk of developing prostate cancer.

Compared with chronic occupational exposure from pesticides, the amount of estrogen that any individual pill-taking woman might excrete is very small. But on a population level, the effect could be significant. “When millions of women are doing it and for a long period of time, it may cause low environmental estrogen levels,” Margel told Canada’s Globe and Mail. “We think further research is needed to explore both oral contraceptives, but also other estrogenic compounds that may contaminate our environment and may cause and increase the incidence and mortality from prostate cancer.”

The new study relied on 2007 data from the International Agency for Research on Cancer and the 2007 United Nations World Contraceptive Use report to determine both pill use and prostate cancer incidence and deaths in 87 countries.

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The authors found no association between prostate cancer rates and the use of other types of nonhormonal birth control, including IUDs, condoms and diaphragms. They also found that the link between birth control pills and prostate cancer risk held up after taking into account a country’s wealth; that’s significant because people in wealthier, more developed countries, such as in North America and Europe, tend to be more likely to use oral birth control and are also more likely to be screened and treated for prostate cancer.

The current study is the first to look at the potential association between birth control pills and prostate cancer, but it stops far short of proving a causal link. The authors acknowledge that their study raises many more questions than it answers. “As such, it must be considered hypothesis generating,” the authors wrote.

In future research, the authors plan to test drinking water for levels of estrogen and to look for estrogenic compounds in cancerous and healthy prostate tissue. In the meantime, they advise women to stay on their chosen form of contraception. “Women should not be throwing away the pill because of this,” stressed co-author Fleshner.

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Meredith Melnick is a reporter at TIME. Find her on Twitter at @MeredithCM. You can also continue the discussion on TIME’s Facebook page and on Twitter at @TIME.

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