Birth control pills have dramatic anti-cancer benefits

Women now have a surprising new reason to go on the pill.

Birth control pills have been around since the 1960s, when they offered women a revolutionary degree of control over their reproductive capabilities. Over the years, the formulation of "the pill" has changed, but it remains one of the most widely used, and most effective, forms of pregnancy prevention.

This week, a very large new study in The Lancet Oncology reported that use of birth control pills provides a significant, and surprisingly large, reduction in the risk of endometrial cancer. The benefit lasts for decades–women who used the pill in the 1960s have the same reduction in cancer rates as women who took it more recently. This is very good news for women.

The new study combined data from 36 earlier studies covering a total of 27,276 women with endometrial cancer and 115,743 without it. The authors–a large group called the Collaborative Group on Epidemiological Studies on Endometrial Cancer–have been working for ten years to collect and analyze this massive data set. Overall, they found that the risk of endometrial cancer in women who had used the pill was only 69% of the risk in women who had never used it. The benefit increased with longer usage: for every 5 years on the pill, women had a 24% reduction in the relative risk of cancer.

Relative risk of endometrial cancer (red line) based on how many years a woman used birth control pills. From "Endometrial cancer and oral contraceptives: an individual participant meta-analysis of 27,276 women with endometrial cancer from 36 epidemiological studies", published online 4 August 2015 in The Lancet Oncology.

Endometrial cancer is the most common gynelogical cancer, accounting for 6% of all cancers in women. The National Cancer Institute estimates that in the U.S., 54,870 women will be diagnosed with endometrial cancer in 2015, and 10,170 will die.

To put the benefit of birth control pills in numeric terms, the new study reports that:

"In high-income countries, 10 years use of oral contraceptives was estimated to reduce the absolute risk of endometrial cancer arising before age 75 years from 2.3 to 1.3 per 100 women."

The benefit is even larger for women who used the pill longer: after 15 years the risk of cancer drops to 1%. The authors estimate that over the past 50 years, the pill has prevented 400,000 endometrial cancers in western Europe, the U.S., and Australasia, including 200,000 in the past decade.

The policy implications of this new study are profound, and likely to be controversial. Women have long struggled to control their reproductive rights, and in many countries access to the pill is still very limited. Even in the U.S. the fight for access to birth control continues: just last year, the Supreme Court ruled in favor of a private corporation, Hobby Lobby, that claimed it had religious objections to providing birth control as part of its health care coverage. (Never mind the absurdity of the notion that a corporation could claim to have religious views.)

Now there's a new and very different reason to provide birth control pills as part of health care coverage. Time will tell whether this dramatic cancer prevention benefit will trump the objections of those who want to deny women access to the pill.