Since the 1960s, women have been using hormonal birth control to control their fertility, despite the fact that scientists still are not sure about its long-term effects.
Two years ago, a massive study conducted in Denmark found that exposure to low-doses of hormonal birth control directly causes depression. The research justifiably made headlines; even a small link would be worrying for women who expect to use hormonal birth control the majority of their adult lives.
New research makes things even more confusing. On Feb. 26, researchers from Ohio State University published a review of 26 studies spanning the last 30 years, finding that progestin birth control didn’t appear to be linked to depression among women at any age.
The Danish study examined all types of hormonal birth control, some of which include a mix of estrogen and progestin. This new review focused specifically on progestin-only forms of birth control. (Estrogen prevents the body from releasing an egg to be fertilized; progestin thickens the cervical mucus,making it impossible for sperm to reach an egg.)
“I think this [study] should provide reassurance for [women], that a progestin-only contraceptive choice is unlikely to cause them to develop depression,” says Brett Worly, a gynecologist at Ohio State University and lead author of the study. It also “suggests that those with previously diagnosed depression should not get worse due to progestin-only contraception.”
Worly and his team combed through all the research they could find regarding progestin birth control and increased risk of depression, spanning large geographic areas with women who had different kinds of access to health care. Worly says that this variety makes their work more robust than the Danish paper.
That said, the review is still far from being able to establish clear guidance. The authors note that none of the studies they could find were randomly controlled trials. The majority of those included in the review were of “moderate” quality, the authors write, meaning there was some risk of bias. In addition, during their research, the team did identify a few smaller studies that found a correlation between progestin use and depression—but none of these studies offered strong enough evidence to fit into their final conclusion.
Furthermore, this research says nothing about the risk of breast cancer and progestin birth control. Last year, the same group of researchers in Denmark who found the link to depression found a small but significant increased risk of developing breast cancer in women who use any kind of hormonal contraception.
For women considering hormonal birth control, progestin-based ones may be less likely to lead to depression than ones that also include estrogen. But given the inconsistency within the field so far, the only thing that’s truly certain is the need for better long-term, double-blind studies—but to date, says Worly, there hasn’t been the level of funding and meticulous record-keeping needed to make that happen.