Ask the Doctors

Dear Doctor: What exactly is the connection between birth control pills and breast cancer? Is the evidence credible? If so, why do so many women still take them -- and so many doctors prescribe them?

Dear Reader: The reason women take them, and doctors prescribe them, is quite simple: Hormonal contraceptives (birth control pills) are an effective means to prevent pregnancy. They are widely available, used by an estimated 140 million women worldwide. This equates to 13 percent of women ages 15 to 49.

The hormones in these pills are either estrogen and progesterone or progesterone alone. Concern about the possible connection to breast cancer centers on the fact that many types of cancer are stimulated by estrogen and progesterone. So hormone-containing pills could theoretically increase the risk for breast cancer.

Older studies assessing this risk have shown either a very mild increase in risk or no increase. A 1996 article in the journal Lancet combined data from 54 studies in 25 countries assessing the risk of breast cancer among women who had used hormonal contraception of various types, including pills, injectables and IUDs. Overall, the study found a small increased risk -- 7 percent -- of breast cancer among women who had ever used hormonal birth control. Among current users, it found a 24 percent increased risk. In women who had stopped taking hormonal birth control for either one to four years or five to nine years, it found a 16 percent and 7 percent increased risk, respectively. No increased risk was found among women who had stopped taking hormonal birth control more than 10 years prior.

Then came a 2017 study in the New England Journal of Medicine of 1.8 million women, ages 15 to 49, in Denmark. The women were followed for an average of about 11 years and, over that timeframe, 11,517 were diagnosed with breast cancer. Those who had ever used hormonal birth control (oral or implantable) had a 20 percent increased risk of breast cancer when compared to women who had never used it. Those who had used it for less than one year had a 9 percent increased risk over never-users, and those who had used hormones for more than 10 years had a 38 percent increased risk.

The risk did not vary based on the type of oral contraceptives, nor was there a difference in risk between oral contraceptives and hormone-containing implantable birth control. When the authors analyzed the risk posed specifically by oral contraceptives, they found an increased risk of 24 percent compared to non-users of any hormonal contraceptive.

A natural question is: What if a woman has an increased risk of breast cancer already? A 2013 study combined data from eight studies of women who were carriers of a BRCA gene mutation that increases the risk for breast cancer. It found a 21 percent increased risk of breast cancer among oral contraceptive users, meaning the risk was no different for women with a higher risk of breast cancer.

Here's another way to look at it: The number of breast cancers caused by hormonal contraceptives is one extra breast cancer case per year (above what already would be expected) for every 7,690 women.

In short, although oral contraceptives may slightly increase the risk of breast cancer, the risk isn't enough to recommend that women stop using a reliable, largely safe method of birth control.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o Media Relations, UCLA Health, 924 Westwood Blvd., Suite 350, Los Angeles, CA, 90095. Owing to the volume of mail, personal replies cannot be provided.)

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