The following originally appeared on The Upshot (copyright 2017, The New York Times Company).
I have a severe intestinal condition known as ulcerative colitis. For some time, I’ve taken an immunosuppressant to control the flare-ups. Like all drugs, this one comes with side effects. While I’m on it, I have an increased risk of developing myelosuppression, a condition in which my bone marrow might stop producing blood cells and platelets. I also have an increased risk of developing skin cancer.
I know all of this, and I take the drug every day. Why? Because the benefits still outweigh the harms. I’ve been in remission from the colitis for years, which lowers my risk for colon cancer. And I don’t worry that I’m going to lose control of my bowels at any moment the way I used to.
Yes, the risks of my medication involve serious diseases like cancer. But you can’t look only at one side of the equation. You can’t look at individual harms alone and make good health decisions.
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.)
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.
Additionally, women have to make sure not to view the breast cancer risk in a vacuum. As I’ve written before, other cancers also matter. Besides breast cancer, there’s evidence that hormonal birth control is associated with higher risks of liver cancer. But it’s also associated with lower risks of ovarian, endometrial and colorectal cancer.
If we look at all cancers together, the benefits seem to outweigh the harms. In other words, a woman who uses hormonal birth control might be less likely to develop cancer, in general, than a woman who doesn’t.
It’s important to note that these statistics assume an average risk of any one cancer or another. Should women have family histories or genetics putting them at higher or lower risks for a particular cancer, the calculus might change.
And let’s not forget that there are many noncancer-related benefits to birth control use. They can be helpful for women who have painful or very heavy periods. They can also be invaluable in family planning. Long-acting reversible contraception, praised for its convenience because it works for an extended time frame without needing any action, can save thousands of dollars over five years compared with other methods.
Every woman should discuss both the positives and the negatives of birth control with her health care provider. The risks include not only those discussed here, but also blood clots and stroke in women who smoke. They also include potential mood changes.
But there are also significant benefits, and for many women these will be greater than the harms. Any discussion that focuses only on one side, or just one aspect of one side, does women a disservice.