More than a quarter of U.S. women in a study by Medco took a medication to treat depression, anxiety or attention deficit disorder in 2010, an increase from a decade earlier and a higher rate than men.

The big pharmacy-benefits manager analyzed a sample of more than two million patients' pharmacy records and found 26% of women took drugs to treat mental-health issues in 2010, up from 22% in 2001. Just 15% of men took these medications in 2010, up from 12%. The results were presented Saturday at a women's-health conference in Washington.

It isn't 100% clear what's behind the increase and the widening gap between the genders, but they indicate "something is going on," says Dr. David Muzina, national practice leader at Medco's Neuroscience Therapeutic Resource Center.

One possibility, he says: Women may be more likely than men to seek outpatient mental-health care. They might also be more vulnerable to depression or anxiety. In addition, some blockbuster antidepressants such as Celexa or Prozac have become available in cheaper, generic form over the last ten years, broadening the number of people able to afford them, he notes.

But while they may be more likely to seek medical help than men – and take more medications overall – women with some serious conditions are also more likely to stop taking medications or fail to properly manage their own health, according to a separate study.