ATLANTA - Scientists studying female sexuality have offered a possible biological explanation for the common complaint that oral contraceptive pills, used by one in four U.S. women at any given time, can dampen desire.
Although the research is preliminary, two small studies presented here last week found that the pill reduces testosterone levels, which in turn may contribute to loss of libido.
One researcher said the problem persists for some women even after they stop taking the pill.
Low libido is the most frequent sexual complaint among U.S. women.
A large 1999 study found that one-third of women age 18-59 reported a persistent lack of interest in sex.
Many factors, both psychological and biological, affect libido. But doctors have long suspected birth-control pills may be one culprit.
The two studies, reported at the annual meeting of the International Society for the Study of Women’s Sexual Health, were designed to investigate that possible link.
In the larger study, doctors at several sites enrolled 106 healthy women, ages 22-50, who complained of loss of sexual desire. All were in stable heterosexual relationships, and none had medical or psychiatric problems that could affect desire.
In addition, all the women had a history of adequate desire and sexual functioning before their symptoms began.
The study subjects were divided into two groups: 43 who were on the pill, and 63 who were not.
When their hormone levels were tested, researchers found the pill group had significantly lower levels of testosterone.
The pill-takers also had significantly higher levels of sex hormone binding globulin, or SHBG, which traps testosterone and keeps it from circulating freely in the bloodstream.
Dr. Anita Clayton of the University of Virginia, who presented the results last Saturday, said the hormonal effects of oral contraceptives may lead to decreased desire,'' and doctors should ask about contraceptive use when female patients complain of low libido. Dr. Irwin Goldstein of Boston University, where the second study was conducted, was more blunt. Find another form of contraception,’’ he said.
Goldstein’s study included 45 women with diminished desire, 21 of whom were taking birth-control pills.
Its results were nearly identical to those of the other study. But Goldstein, who heads the university’s Institute for Sexual Medicine, said he has seen something even more ominous in his clinic.
Out of 100 patients who stopped taking oral contraceptives, one-third still had elevated SHBG levels up to a year later, suggesting the imbalance might be permanent.
Dr. Alan Altman of Harvard Medical School, who was not involved in either study, said: ``We assumed that oral estrogen (in birth-control pills) would increase SHBG, which would lower available testosterone. But it was important to see if we could validate that assumption.’’
The two studies bolstered the idea that low libido may be linked to a woman’s hormone levels, especially testosterone. But several researchers pointed out that the link has yet to be demonstrated conclusively. Moreover, lack of sexual desire is often the result of social or psychological factors, including a history of abuse, unyielding demands of work and homemaking, or an inept partner.
Goldstein acknowledged that a much bigger study involving women with and without sexual dysfunction, on and off birth-control pills, would be needed to prove the association between the pill, low testosterone and lack of desire. Obtaining money for such a study from the government or drug companies has been next to impossible, he said.

While some researchers in the fledgling field of female sexual dysfunction study hormone levels and drug effects, much of the annual meeting was devoted to non-medical questions and methods used in traditional talk therapy.
Two studies found that non-physical interventions - teaching women about their sexual responses or asking them to imagine more positive feelings about their sexuality - resulted in improved physical arousal.
In one, Stephanie Kuffel of the University of Washington examined the relationship between depressive thinking and sexual arousal.
Her study involved 56 healthy women ages 21-49, half with normal moods and half with depressed moods, though not clinically depressed. All the women were given both positive and negative mental scenarios and exposed to an erotic video segment after each one. The researchers measured physical arousal (blood flow to the genitals), as well as subjective arousal (how turned on'' the subjects felt). In the positive scenario, the women were asked to imagine that sex was a very important and pleasurable part of their lives, that they liked to initiate sex and were easily aroused. In the negative scenario, they were told: You don’t really enjoy sex. You have little sexual desire, and you almost never initiate sex. It is difficult for you to become aroused.’’
The women with depressed moods had lower desire at the start of the experiment. But all the women had higher subjective and physical arousal when asked to adopt the positive mindset.
Kuffel cautioned, however, that what happened in the laboratory might not work in the real world.
It's possible that medical problems or relationship issues could override'' the effect of positive thinking, she said. Sexual response is multifactorial.’’