Does birth control cause brain cancer?

Today’s scary headline is about birth control increasing the risk for brain cancer. Let’s take a look at the data behind the headlines. (It’s a lot less scary than it sounds.)

First, some definitions

Glioma is a general term for several kinds of brain tumors. Together, these tumors make up much of the brain cancer diagnosed in the United States. All of these cancers become more common as people get older, and men are more likely than women to be diagnosed with them. Scientists are still working on understanding how these cancers start. They know that being exposed to radiation increases the risk, as do some rare genetic conditions.

What did this study show?

It’s important to know that these cancers are super rare among young women. In the U.S., fewer than 1 in 100,000 women under age 40 are diagnosed with glioma each year. This study showed that Danish women who had used the pill or hormonal IUD were about 50% more likely to be diagnosed with a brain tumor. But glioma is so rare that even with a 50% higher likelihood, when you combine it with the total number of women diagnosed, it still means that fewer than 1 in 100,000 women under age 40 are diagnosed each year.

And here’s the tricky thing about this kind of study: women who use birth control also have other things in common. So it’s hard to say whether it’s the birth control or the other things that cause the relationship that researchers see. For example, women who use birth control are likely to have fewer children than those who don’t. Other research has shown that the number of children a woman has is associated with brain cancer. Plus, women who use progestin-only types of birth control may have medical reasons why they can’t use combined hormonal contraception—such as a family history of blood clots, obesity, or smoking—which are also associated with cancer.

And the moral is…

When you see a scary headline, always look for information about the total number of people affected before worrying too much. That information can make a big difference. And as always, if you’re concerned about your health and birth control, talk to your health care provider!