It's no secret that women who take a hormonal birth control pill have less interest in sex. One theory as to why is that combination birth control pills ― which contain a synthetic form of estrogen and progestin ― lower the levels of circulating testosterone in the body, thus lowering women's libidos. But a new study published in the December issue of the American Journal of Obstetrics & Gynecology suggests that the makeup of the Pill may not be the culprit after all.
Researchers from Ohio State University College of Medicine tested 50 sexually active women who used either a combined oral contraceptive or a progestin-only injection. The combination birth control pill is thought to dampen libido in part by increasing the production of sex hormone binding globulin in the liver, a protein that binds to and inactivates circulating testosterone in the blood. In contrast, progestin-only injections, which lack estrogen, are thought to avoid this effect. (More on Time.com: )
The women filled out a Female Sexual Function Index (FSFI), a questionnaire designed to measure sexual desire and satisfaction. They also had their testosterone and estrogen levels checked.
Although free testosterone levels were significantly higher among the women using the injection, there was no significant difference between the two groups' scores on sexual desire or satisfaction. The women's scores on the FSFI indicated high levels of sexual function, but the study did not compare their scores to those of a control group of women who did not use hormonal birth control. (More on Time.com: )
Women who used the Pill were on average better educated and more likely to have never been pregnant or had children, compared with those using the progestin-only injection. They were also more likely to be white. Both groups had steady sexual partners, were of similar age, had used contraception for about the same amount of time (an average of 4 to 5 years) and had similar religious affiliations. The researchers controlled for differences like having children, which can affect sex drive. (More on Time.com: )
The new study doesn't clear up the mysterious link between diminished sexual satisfaction and oral contraceptives, but it does suggest the possibility that there may be something else about some long-time users of birth control ― oh, say, boredom with the same old partner? ― that could help explain it.