A vaginal gel that sharply reduces a woman’s risk of infection with the AIDS virus is even more effective against genital herpes, a much more common risk for young American women, a new study has found.

The study, by researchers from the National Institutes of Health, Gilead Sciences Inc. and universities in Belgium and Italy, suggests that the microbicide gel, which was originally developed to fight AIDS in Africa, could lower the incidence of herpes in many women.

“This could be incredibly helpful,” said Dr. Jeanne Marrazzo, a herpes expert from the University of Washington’s medical school. “Protection that a woman can control is the holy grail in this field. It’s hard for me to believe that something that protects against both H.I.V. and herpes wouldn’t be appealing to a lot of young American women.”

An executive at Gilead, the company that makes tenofovir, the anti-AIDS drug that is the gel’s active ingredient, said the company was debating whether to spend the millions of dollars needed to get the gel approved for the American market. Even if the company pressed ahead immediately, “it would be three to four years before we were ready to submit data” to the Food and Drug Administration, Norbert W. Bischofberger, Gilead’s chief scientific officer, said.