In a landmark study that may have lasting repercussions for the estimated 80 percent of sexually active young women in the United States use hormonal contraceptives during their reproductive years, depression has been linked to hormonal contraception.

The results have been published in JAMA Psychiatry.

The research team studied the the medical records of over one million healthy Danish women between the ages of 15 and 34 who had been taking hormonal contraception between 1995 and 2013. Women who used hormonal contraception were associated with an increased risk of depression compared to non-hormonal contraception users, as gaged by how many of these women were later prescribed an antidepressant for the first time, or received a formal diagnosis of depression from a psychiatric hospital.

“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,” the study concluded.

Depression rates varied by the type of hormone, how it was administered, and by age of the woman. Highest rates of women filling their first prescription for an antidepressant were among women using medroxyprogesterone acetate depot, contraceptive implants and the norgestrolmin transdermal patch. Adolescents using hormonal contraception were associated with higher rates of depression than adult women. Women on the most popular type of birth control – combined oral contraceptives – were 23 percent more likely to be prescribed antidepressants than non-users.

It's all likely an emotional reaction. The scientists pointed towards recent research that has suggested the link between hormonal contraception and depression may center around the role of estrogen and progesterone and these hormones' influence on the cortical and subcortical regions of the brain, which are responsible for emotional processing.

"Because mood symptoms are a known reason for cessation of hormonal contraceptive use [and] hormonal contraception introduces synthetic hormones and modulates the internal hormone production, an examination of the influence of hormonal contraceptives on women's mood is warranted," they wrote.

The notion that hormones can influence depression risk is, of course, nothing new. Women are twice as likely as men to be diagnosed with depression due to the ramped-up production of progesterone that first begins in puberty. Other research has shown that women with higher levels of progesterone and lower levels of estrogen may be at increased risk of depression.