Study co-author Dr. Oevind Lidegaard, of the University of Copenhagen in Denmark, and colleagues report their findings in JAMA Psychiatry.

According to the most recent data from the Centers for Disease Control and Prevention (CDC), in the United States, more than 61 percent of women of reproductive age currently use some form of contraception.

Oral contraceptives - or birth control pills - remain the most common form of contraception, currently used by around 16 percent of women aged 15-44 in the U.S.

Combined oral contraceptives contain the hormones estrogen and progestin; progestin may refer to synthetic forms of the hormone progesterone or progesterone itself. These contraceptives work by preventing ovulation and making it harder for sperm to reach the egg, thereby preventing pregnancy.

The progestin-only pill - also known as the "mini-pill" - primarily prevents pregnancy by stopping sperm from reaching the egg, and it may sometimes suppress ovulation.

Other forms of hormonal contraception include the birth control implant, injection, patch, and the vaginal contraception ring - all of which release estrogen, progestin, or both as a way of preventing pregnancy.
Limited data on how hormonal contraception influences depression

Dr. Lidegaard and team note that previous research has suggested that changes in estrogen and progesterone levels - particularly the latter - may play a role in depression. However, they say few studies have investigated whether the use of hormonal contraceptives influences depression risk.

"The aim of this study was to assess the influence of specific types of hormonal contraceptives on the risk for first use of antidepressants and first diagnosis of depression as an inpatient or an outpatient at a psychiatric hospital," say the authors.

To reach their findings, the team analyzed 1995-2013 data from the Danish National Prescription Register and the Psychiatric Central Research Register, including more than 1 million women aged 15-34.

Over an average follow-up period of 6.4 years, the researchers assessed participants' use of hormonal contraception and tracked first diagnoses of depression and first-time antidepressant use. None of the participants had a history of depression or antidepressant use at study baseline, the researchers report.

During follow-up, 55.5 percent of the subjects were current or recent users of hormonal contraceptives, and there were 23,077 first depression diagnoses and 133,178 first-time antidepressant prescriptions.
Depression risk greater for teens who use hormonal contraceptives

Overall, compared with nonusers of hormonal contraceptives, participants who were current or recent users of hormonal contraceptives were found to be at greater risk for depression and antidepressant use.

As an example, women aged 20-34 who used combined oral contraceptives or the progestin-only pill were at 1.23 and 1.34 times greater relative risk of first-time antidepressant use, respectively, and similar or slightly lower estimates were found for the risk of depression diagnosis.

The risks were higher for adolescents aged 15-19, the team found; use of combined oral contraceptives or progestin-only pills among this age group was associated with a 1.8 and 2.2 times greater relative risk of first-time antidepressant use, respectively, and those who used non-oral hormonal contraceptives were at three times greater risk.

Again, similar or marginally lower estimates arose for adolescent girls when it came to the risk of depression diagnosis.

"Our data indicate that adolescent girls are more sensitive than older women to the influence of hormonal contraceptive use on the risk for first use of antidepressants or first diagnosis of depression," the authors note. "This finding could be influenced by attrition of susceptibility, but also that adolescent girls are more vulnerable to risk factors for depression."

Overall, the researchers conclude that their findings suggest the use of hormonal contraceptives may raise the risk of depression and antidepressant use for teenage girls and women.

    "In this study, use of all types of hormonal contraceptives was positively associated with a subsequent use of antidepressants and a diagnosis of depression.

    That finding complies with the theory of progesterone involvement in the etiology of depression, because progestin dominates combined and progestin-only contraceptive."
'It is important for women to be aware of potential risk'

Talking to Medical News Today, Dr. Lidegaard said it is no surprise that hormonal contraceptives might raise the risk of depression.

"Generally estrogen improves mood and progestogens worsen mood. As hormonal contraception generally is dominated by progestogens, it is not very surprising that mood generally has a tendency to be changed in direction for depression," he said.

Still, despite their findings, Dr. Lidegaard told MNT he does not recommend that women of reproductive age turn to non-hormonal forms of contraception.

"But I think it is important that women, especially young women, are aware of this potential risk with use of hormonal contraception, including oral contraceptives," he added.

The team now plans to investigate whether the use of hormonal contraceptives is associated with increased risk of suicide or suicidal attempts.