Postmenopausal women who have lost interest in sex may be able to bring their libidos back to life with a testosterone patch, according to new research published this week in The New England Journal of Medicine.

Even though results were modest, one expert called them meaningful.

However, the use of the male hormone to boost sex drive in women may not be risk-free. Out of the 814 women in the study, four women who were taking testosterone developed breast cancer, but none of the women on placebo did. It's not clear whether this was a statistical blip or a warning sign that excess testosterone could cause or spur the growth of a malignancy. Some women also reported excess hair growth, although none stopped using the hormone for this reason.

Susan R. Davis, M.D., Ph.D., of Monash University in Australia, and colleagues in the United States, Canada, and Sweden, evaluated two different doses of testosterone delivered by Procter & Gamble Pharmaceuticals' Intrinsa patch. In 2004, a U.S. Food and Drug Administration (FDA) panel gave Intrinsa the thumbs down and called for larger, longer studies to ensure that the medication was safe, in addition to proving that it actually helped women's sex lives.

As the new findings show, it did. Wearing the higher-dose testosterone patch boosted a woman's "satisfying sexual experiences" by an average of 2.1 times every four weeks, compared to an increase of just 0.7 such experiences for women taking a placebo. Both testosterone doses used in the study seemed to increase desire and decrease distress.

"Although the change in activity is modest, that's something that is appropriate and I think most women would be more than happy with it," says study co-author Sheryl A. Kingsberg, Ph.D., chief of behavioral medicine at University Hospitals Case Medical Center in Cleveland, Ohio. "They wanted to return to the level of desire they had in their premenopausal years, and that's what they got." Before starting treatment, the women in the study had been having satisfying sex about twice a month on average, Kingsberg points out; the higher-dose patch increased that to once a week.

"For most women and providers of health care for women, that modest benefit is clinically meaningful," agrees North American Menopause Society president JoAnn V. Pinkerton, M.D., a professor of obstetrics and gynecology at the University of Virginia, in Charlottesville, who did not participate in the study.

Some women lose interest in sex during and after menopause, due in part to the drop in estrogen that comes with the "change of life." While taking estrogen can increase lubrication and possibly restore a woman's sex drive, hormone replacement is now understood to raise the risk of heart disease and stroke. Many physicians prescribe testosterone instead, although there is currently no testosterone product that's FDA-approved for treating women with "hypoactive sexual desire disorder." The European Union has approved Intrinsa, but only for women who have had their ovaries removed, a procedure also known as surgical menopause. Read more on the medical reasons why some women don't want sex

In the current study, 814 women who had undergone either surgical menopause or natural menopause were randomly assigned to use daily a placebo patch or an Intrinsa patch containing either 150 or 300 micrograms of testosterone. The trial lasted for a year, and a subset of women was followed for an additional year. Procter & Gamble Pharmaceuticals sponsored the study and helped design the trial as well as collect and analyze the data.

"Based on these data and other studies we've conducted, we are continuing our talks with [the] FDA to explore new opportunities and pathways forward," says Procter & Gamble spokesperson, Tom Milliken.

One of the women on the 300-microgram dose was found to have breast cancer three months after the study ended; three others in the testosterone groups learned they had the disease between four and 12 months after treatment began.

"We do not know if the testosterone patch contributed to proliferation or metastasis of the breast cancer in women diagnosed in the earlier months of the study, potentially affecting their long-term survival," says Leslie R. Schover, Ph.D., a behavioral scientist at the University of Texas M.D. Anderson Cancer Center, in Houston, who recently wrote an article analyzing research on testosterone for low libido. "A valid safety study needs at least a five-year follow-up period in a large, randomized trial. If women use Intrinsa without such a trial, I believe they are risking their lives to gain a very modest increase in sexual desire." Learn why birth control is safer than ever (and sometimes it's even good for you)

But Dr. Davis says she is not concerned about the increased breast cancer risk seen in the study. Four breast cancer diagnoses among 814 women during a two-year period "is not unexpected," she says, and given that twice as many study participants were taking testosterone than were on placebo, "it is probably a chance finding that they were in the two treatment groups."

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Many doctors who treat postmenopausal women -- and prescribe testosterone off-label to some of them -- say a treatment tailored to women is sorely needed and would probably be safe with short-term use. "We don't have enough safety data to say it's safe for long-term use, but I think short-term, the benefits clearly outweigh the risks," Dr. Pinkerton says. Read more: 10 questions to ask a new partner before having sex

But some experts warn that a pill or patch isn't always the answer to a sexual problem.

"For women there are so many other things that can contribute to sexual issues, starting from the fact that the most important sex organ is the brain," says Marcie Richardson, M.D., director of the Harvard Vanguard Menopause Survey in Boston. "I'm glad that people are trying to sort this out with good objective evidence, but I hope we don't fall victim to the notion that this is all about medication, because it's not."

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All About Sexual and Reproductive Health • Food and Drug Administration