Viagra is well established for treating male impotence. A new study slated to appear in the Journal of the American Medical Association suggests the drug can also relieve some sexual difficulties in women caused by antidepressant use.

Women and men taking antidepressants called serotonin reuptake inhibitors sometimes experience a fading libido. An estimated 30 to 70 percent of people taking these antidepressants register sex-related complaints at some point. SRIs include Prozac, Paxil, Zoloft, Lexapro, Celexa and Anafranil.

In women, this change can be compounded by decreased genital sensitivity, vaginal dryness, delayed or absent orgasms and general dissatisfaction with sex.

Viagra, also called sildenafil citrate, has been a blockbuster drug for men with sexual dysfunction and for its maker, Pfizer Inc. But Pfizer largely gave up on testing Viagra in women four years ago after thousands of women receiving it had failed to register much effect.

The company did continue to fund research for certain subgroups of women for whom the drug might still have potential, including those in the new study who were taking SRI antidepressants.

In men, Viagra boosts the natural effect of nitric oxide, which induces blood vessels to relax and facilitates blood flow to the penis, causing an erection. In women, blood vessels in the vagina and clitoris also swell in response to the drug, but studies in women had failed to show clear gains in sexual function.

Viagra doesn’t directly enhance libido. Scientists have suggested that the drug didn’t work on women because their cascade of arousal, desire and orgasm is more complicated than men’s.

Indeed, the results of this study might not be applicable to other women, the authors say. It remains unclear why Viagra would work for women taking anti-depressants, but not for other women. “The bottom line is we don’t know for sure,” says study coauthor Julia Heiman, a clinical psychologist who is director of the Kinsey Institute at Indiana University in Bloomington. But these women might have been more motivated than women in previous studies. “We were giving this drug to women who wanted this to change,” she says.

Using newspaper advertisements, postings and referrals, Heiman and her colleagues recruited 100 women, ages 18 to 50, who reported having sexual difficulties while on an SRI. None had pre-existing sexual troubles. The researchers randomly assigned half of the women to get Viagra and half to receive a placebo. They instructed the women to take a pill one or two hours before having sex.

The women recorded their experiences in diaries and each woman met with a researcher four times during the eight-week study, including visits at the start and finish. These discussions and the diary entries enabled doctors, using a standardized set of questions about sexual interactions, to come up with a composite score of sexual function for each woman before the study and after the eight weeks had elapsed.

While the women taking placebos registered only a very slight improvement overall in benchmarks of sexual function, women receiving Viagra reported significant gains, the researchers report in the July 23/30 JAMA. In particular, the women said their ability to reach orgasm and their orgasm satisfaction improved markedly. Other aspects of sexual function — arousal, desire and natural vaginal lubrication — improved less.

The work represents the first randomized trial to show a positive effect from Viagra in women with SRI-linked sexual problems, the researchers note. Earlier studies in which participants knew they were receiving the drug had also suggested Viagra might work in this group.

“This study doesn’t come completely out of the blue,” says John Markowitz, a psychiatrist at the New York State Psychiatric Institute in Manhattan. The findings reflect a clinical concern that doctors have with these anti-depressants. Sexual dysfunction “is probably the Achilles heel of SRIs,” Markowitz says. Although Viagra isn’t approved specifically to be prescribed for women, he says, “doctors have been doing it for a long time. This provides some evidence to back up what I suspect is a widespread practice.”

Women who experience sexual side effects while taking antidepressants are three times as likely to stop taking SRIs as are other women on these antidepressants, previous research showed. Women participating in the new trial continued to take SRI antidepressants during the eight-week test period.

Heiman cautions that the trial was relatively small with significant but modest effects. It doesn’t suggest a broad new standard for women who have sexual troubles. “For this subgroup of women, this approach could be somewhat helpful, and could be enough to make a difference,” she says.

Meanwhile, other studies continue to search for a “pink Viagra,” centering on women’s use of testosterone patches, a combination estrogen-testosterone pill, and Wellbutrin, an antidepressant that acts differently from the SRIs.

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