From NPR:

Today, millions of women around the world use hormonal contraceptives that have expanded beyond the Pill to patches, implants, injections and uterine devices. Decades of research support their safety, and serious but very rare side effects such as blood clots are finally much better understood. But other areas of research lag, and we still don’t know as much as we’d like about how these medications affect women’s mental health.

So when a study came out linking hormonal birth control and depression, the headlines went wild. The stories made for good clicks, but not so great science reporting. Insufficient skepticism about a single study makes it easy to imply birth control definitely causes depression when the study shows nothing of the sort.

The study, which was conducted in Denmark and published in JAMA Psychiatry, analyzed 14 years’ worth of health data for more than 1 million women from national healthcare systems and databases not available in most other countries. It also measured depression two ways: diagnosis at a psychiatric hospital, which would be quite severe depression, or filling a prescription for antidepressants. Across the whole study, 2 percent of all women ages 15 to 34, were diagnosed with depression at a hospital and 13 percent began taking antidepressants.

Several news stories reported an 80 percent increase in risk of depression in some groups of women, but few noted that was relative risk, which is an expression of proportional increase. An 80 percent relative risk does not mean that 80 percent of women taking hormonal birth control develop depression. It means that if 10 women not taking hormonal birth control develop depression, then 18 women on the birth control will develop depression.

In this study, that 80 percent increase in relative risk referred specifically to the likelihood that those ages 15 to 19 …

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