I bring all of this up because this past week there were big headlines about a new study that linked contraceptive pills and other hormonal birth control to an increased risk of breast cancer. Some news articles stressed the risk of a commonly used medication. Others, like the one by Roni Caryn Rabin in The New York Times, carefully placed the numbers in context to explain that the absolute risk is very small. (Relative risk is the percentage change in one’s absolute risk as a result of some change in behavior.)

Image The small increased breast cancer risk from birth control pills and intrauterine devices should be weighed against potential benefits. Credit... Theo Stroomer for The New York Times.

I would go even further. This was a prospective cohort study, meaning it was an observational study that followed women over time and saw what happened to them naturally. The data set didn’t allow for adjusting for some factors that could also be associated with breast cancer, like age at first menstruation; whether women breast-fed; whether they consumed alcohol and how much; whether they were physically active; and more. The study found only an association, and not causal proof you might obtain from a randomized controlled trial.

Even if we accept the findings in full, we might expect an additional 13 cases of breast cancer for every 100,000 person years of use. Another way to say that is for every 7,690 women who use hormonal contraception for one year, one extra might get breast cancer. The rest would not be affected.

That’s a very small risk. Moreover, it’s for women over all. The risk is different for women of different ages. For women younger than 35, there were only two additional cases for every 100,000 person years of use. That means only one extra case of breast cancer for every 50,000 women 35 or younger who use hormonal contraception each year.

This cancer risk isn’t even a new side effect. We’ve known about the link for years. It was hoped that newer formulations might have reduced it, but that doesn’t appear to be the case. The study further established that this increased risk also holds for the hormone progestin, which is also used in pills and intrauterine devices.