(CNN) Taking hormonal birth control might be associated with an increased risk for depression compared with those who don't use contraception, according to a new study published in the journal JAMA Psychiatry last week.
The finding is something that users have long suspected, as about 30% of women who ever used the pill in the United States eventually quit because of dissatisfaction with side effects, according to a 2013 report from the Centers for Disease Control and Prevention (PDF).
"We have known for decades that women's sex hormones estrogen and progesterone have an influence on many women's mood. Therefore, it is not very surprising that also external artificial hormones acting in the same way and on the same centers as the natural hormones might also influence women's mood or even be responsible for depression development," said Dr. Øjvind Lidegaard, a professor at the University of Copenhagen in Denmark and lead supervisor of the study.
Lidegaard and his colleagues tracked the health of more than 1 million Danish women between the ages of 15 and 34 over 14 years, using data from the National Prescription Register and the Psychiatric Central Research Register in Denmark.
To ensure that depression was properly identified in connection with birth control use, women with a depression diagnosis before their 15th birthdays or the start of the data collection were excluded.
After analyzing the data, the researchers found that the use of hormonal birth control was positively linked to a subsequent depression diagnosis and use of antidepressants.
Among all hormonal birth control users in the study, there was a 40% increased risk of depression after six months, compared to women who did not use hormonal birth control, the researchers found. The mean age of birth control users in the study was 24.
The users of combined oral birth control pills experienced a 1.2-fold higher rate of subsequently taking antidepressants during the study period than those not using the birth control. Women who used progestin-only birth control pills experienced a 1.3-fold higher rate, according to the study.
As for non-oral forms of hormonal birth control, those who used the transdermal patch had a two-fold increased risk and those who used the vaginal ring had a 1.5-fold increased risk.
Similar rate increases were found for depression diagnoses, according to the study.
The researchers noted in their study that the difference in risk rates among women taking non-oral and oral forms of birth control might be due to a difference in dose rather than how the contraceptives are administered.
The researchers also noted that this association does not imply that birth control alone causes depression -- and more research is needed to better understand the possible link.
"Adolescents seemed more vulnerable to this risk than women 20 to 34 years old. Further studies are warranted to examine depression as a potential adverse effect of hormonal contraceptive use," the researchers wrote in their study.
One of the study authors has a history of consulting for two pharmaceutical companies, Lundbeck and AstraZeneca, and another author reported receiving funds for talks from Exeltis. No other conflicts of interest were disclosed.
Lidegaard said the study results could translate to women in the United States.
In the United States, about 62% of women 15 to 44 years old use some form of contraception (PDF).
Among those women, 16% use the pill, 15.5% use female sterilization, and 7.2% use long-acting reversible contraception, such as an IUD or implant, according to the National Center for Health Statistics.
What do other experts think of the new study? Dr. Kathryn Holloway, an ob-gyn practitioner at the Institute for Women's Health in San Antonio, Texas, called it impressive.
However, while there may be a correlation between hormonal birth control with the prescription of antidepressants and perhaps a depression diagnosis, causation is hard to prove, she said.
"Although this study suggests an increased risk of depression with combined hormonal contraception, the increase does not seem so great as to significantly change how I counsel patients," Holloway said, adding that, "Depression is not something to be taken lightly and should not be a missed diagnosis. It is important for physicians to monitor and evaluate for any possible side effects, even if rare, with any prescribed medication."
While birth control comes with some negative side effects, from stomach cramps to increased risk of stroke, it can also provide some health benefits.
Hormonal birth control not only prevents unwanted pregnancies, but also can regulate menstrual cycles, treat endometriosis pelvic pain, control symptoms of fibroids, help acne breakouts, and reduce the risk of some cancers that affect reproductive organs.
There are some forms of birth control that are not hormonal, Holloway said.
"Hormonal contraception options range from the combined oral contraceptive pills to an intrauterine device and a birth control shot such as Depo-Provera," Holloway said.
"Non-hormonal contraception options include condoms, cervical caps, the vaginal sponge and spermicide, and the copper IUD," she added. "Some patients are successful with natural family planning by monitoring their cycle on a calendar or menstrual cycle app. It's up to the patient and their ob-gyn to decide which method of birth control is right for their lifestyle and well-being."