A Danish study linking birth control with depression has generated excited headlines around the world and struck a chord among many women.
But the response to the study is also providing a perfect example of how the public and the media often misunderstand medical and scientific research, experts said Thursday.
Here's what the study did find: Women who used hormone-based birth control, such as the pill, implants or patches, were more likely to be prescribed an antidepressant or to be diagnosed with depression than women who did not use birth control.
What the study did not find was proof that the birth control actually causes depression. Just because two things are linked doesn't mean one caused the other. And the study did not actually find anything that's startling to researchers in the field - they've been suspecting there may be a link for decades.
"This is really resonating with a lot of women," said Chelsea Polis, a senior research scientist at the Guttmacher Institute, which studies reproductive health.
"Part of that has to do with women's individual experiences. Women do feel that clinicians aren't taking their concerns seriously sometimes," she added. "And part of this reaction is this scientific literacy gap."
For the study, Dr. Oejvind Lidegaard of the University of Copenhagen and colleagues looked at the medical records of more than 1 million young women and girls aged 15 to 34 from 1995 through 2013.
"Use of hormonal contraception, especially among adolescents, was associated with subsequent use of antidepressants and a first diagnosis of depression, suggesting depression as a potential adverse effect of hormonal contraceptive use," they wrote in their report, published in the Journal of the American Medical Association's JAMA Psychiatry.
While no single study is ever meant to be the last word on anything, this particular one generated a lot of excitement and dramatic headlines.
"Whenever these studies come out there are these big news reports," said Katherine Keyes, associate professor of epidemiology at the Columbia University Mailman School of Public Health, who also studies links between depression and birth control and who helped review the Danish study for JAMA.
"Feeling Depressed? A New Study Reveals It May Be Your Birth Control", reads one. "'It's not in your head': Birth control may cause depression, new study says", reads another.
The study found that women who used hormonal contraception - mostly birth control pills but also implants and patches - were 40 percent more likely to get a prescription for an antidepressant or to be diagnosed with depression.
Teenagers were 80 percent more likely.
But overall, the rates of depression were still low. Every year, 2.2 out of 100 women using hormone-based birth control were likely to start antidepressants, compared to 1.7 out of 100 women not using them. While the risk is higher, it's a very small risk to start with.
The rates are even closer for women being diagnosed with depression at a psychiatric hospital. The study found that 0.3 out of every 100 birth control users were diagnosed with depression versus 0.28 percent of non-birth-control users.
This does not add up to very many more cases in the birth control group.
"You can have something that is twice the risk, but it can be a very rare outcome," Keyes said.
And the rates dropped after a few years, so that women using birth control were less likely to develop depression.
Nonetheless, Polis said, some of the reaction has been intense.
"Women who feel vindicated by this study have this sense that this has been known all along and they've been hiding this from us," she said.
"And on the other hand, there is a sense that science has not really looked into this before."
In fact, scientists have done many studies and there's been a lot of coverage. The trouble is, the results have been mixed.
Keyes herself did a pair of studies that found just the opposite of what the Danish researchers found. "Women using hormonal contraception had a lower risk of depression," she said.
And that's not surprising. It's how scientific and medical research is supposed to work. One team does a study using certain groups and certain ways of measuring outcomes, and another approaches the problem from a different direction.
"It's not necessarily that the findings stand in stark contrast to each other," Keyes said.
Her team found other associations with birth control besides depression.
"We found women who use hormonal contraceptives are more likely to eat leafy green vegetables once a week. They are more likely to go to a doctor. They are more likely to exercise. They are health-conscious women."
That doesn't mean that contraceptives caused those behaviors, however. Each study adds a piece to a bigger puzzle.
Teens starting birth control may be going through social changes, as well as biological changes, including puberty. Women in their childbearing years may be taking birth control in between pregnancies, which can wreak havoc on mood and hormones.
"There is a tendency to get to the biology a little bit quickly without considering the social lives of women," Keyes said.
That said, the information in the study will guide future research.
"This was a very well-conducted study. I think it's an important piece of information," Keyes said. "It lowers my confidence in my hypothesis, which is that hormonal contraception sort of regulates mood."
It can take years to come to a consensus, medically or scientifically. And birth control is a moving target for many reasons: there are dozens of different formulations, women use birth control for varying reasons, and different women are genetically and biologically different.
There is a reason to suspect a link. "We know the number one reason women report stopping hormonal contraceptives is mood disorders," Keyes said.
"A common complaint of women using hormonal contraception is that they feel irritable, they feel tired, they have weight gain. But that's not clinical depression."
And it's known that hormones - both '"female" and "male" hormones - can act on the brain.
"There has been this tendency, especially when thinking why there are gender differences in depression, to think, 'well, it must be hormones'," Keyes said.
"It's a natural hypothesis, and some of it is rooted in stereotypical images of gender," she added.
"It's a road that goes down a dark place. It takes you to the idea that women can't be in charge because their hormones are all over the place."
But there are two important facts to remember, Keyes said. Birth control is safe and effective for most women, and depression is a serious health problem.
"Hormonal birth control is not only protective against pregnancy but also has other health benefits such as decreased risk of some cancer and other health benefits," she said.
"I don't think this study alone is strong enough to change clinical practice in term of prescribing contraceptives, but it should give doctors pause to monitor their patients closely."
And that's important, too.
"It is really important to point out that depression remains one of the most untreated health problems, especially for women," Keyes said.
"We need to be getting women in to their physicians, getting screened for depression."