Hormonal contraceptives may affect the efficacy of exposure therapy, according to new research from Germany that examined women with spider phobia.

“Considerable evidence from human and animal studies suggests that estrogen mediates the amount and rate of fear extinction learning and its later recall. Oral contraceptives may interfere with these processes due to suppressing endogenous estrogen secretion, thus leading to impaired fear extinction,” the researchers wrote in their study, which was published in the Journal of Psychiatric Research.

For the study, 54 women with arachnophobia received two sessions of exposure therapy in which the participants systematically confronted their fear in a controlled environment. About half of the participants were taking oral contraceptives, while the others were not.

The participants completed three questionnaires on the severity of their fear of spiders before and after the therapy. In order to gain an objective and behavioral measure of phobia, the participants were also asked to get as close as possible to a house spider placed in a plastic container.

Immediately after therapy, most of the participants were able to approach the spider more closely than before and reported lower levels of subjective fear. But during a 6-week follow up assessment, the researchers observed a stronger reduction of subjective fear (but not behavioral fear) among women not taking oral contraceptives compared to women taking oral contraceptives.

The findings indicate “that women taking oral contraceptives tend to profit less from exposure therapy relative to women who do not use oral contraceptives,” the researchers said. The results are mostly in line with a previous study, which also found that spider-phobic women using hormonal contraceptives exhibited a reduced response to exposure therapy.

“The negative influence of oral contraceptives on therapy effectiveness didn’t become evident until six weeks later. Our results show that it is necessary to monitor and consider any potential hormonal impact during psychotherapeutic treatment,” study author Armin Zlomuzica said in a news release.

The researchers also found that women taking oral contraceptives tended to report significantly more severe spider phobia prior to treatment.

“To our knowledge, the effects of hormonal contraceptives on symptom severity in patients with anxiety disorders has not been examined systematically so far. Thus, at this moment, it can only be speculated how oral contraceptives and estrogen levels might affect anxiety levels in clinical populations and how this effect is further modulated during behavioral treatment,” they wrote.

The study, “No pills, more skills: The adverse effect of hormonal contraceptive use on exposure therapy benefit“, was authored by Friederike Raeder, Franziska Heidemann, Manfred Schedlowski, Jürgen Margraf, and Armin Zlomuzica.