By Toni Clarke

WASHINGTON, Feb 11 (Reuters) - The U.S. Food and Drug Administration hit back on Tuesday at critics who have charged it with gender bias for rejecting a drug for low female libido from Sprout Pharmaceuticals.

Women's groups, who have lobbied heavily for the drug's approval, pounced after the FDA denied an appeal to approve the product, a once-a-day treatment for hypoactive sexual desire disorder (HSDD), accusing the agency of bias.

"When it comes to approving drugs for male sexual dysfunction, the FDA says yes with more limited research and serious side effects, but when it comes to women, their go-slow tactics are preventing us from having access to a treatment option where we make the decision in consultation with our healthcare provider," said Terry O'Neill, president of the National Organization for Women, in a statement.

The FDA rejected the charge, saying in an email that it "engaged in a scientific process in which we evaluated whether the drug's benefits outweigh its risk."

The FDA declined to approve the drug, flibanserin, last year saying its effects were "modest" and did not outweigh side effects such as dizziness, nausea and fatigue.

On Tuesday Sprout said the agency had asked for three new studies, two to test its interaction with other drugs, and a third to simulate driving since nearly 10 percent of women in clinical trials became sleepy while taking it.

Sprout, which is privately held, said it welcomed the FDA's guidance and viewed it as "a significant step" toward the once-a-day treatment.

The FDA's decision followed intense lobbying by women's advocates.

In January, Democratic lawmakers Debbie Wasserman Schultz, Chellie Pingree, Nita Lowey and Louise Slaughter - wrote to FDA Commissioner Margaret Hamburg urging the agency to give "careful review" to the flibanserin data and apply "the same standards of consideration given to the approved drugs for men in your risk/benefit evaluation."

On Jan. 28 representatives from eight women's groups, including NOW, the Center for Health and Gender Equity, Jewish Women International and the National Council of Women's Organizations met with Dr. Janet Woodcock, head of the FDA's pharmaceuticals division, and conveyed their view that the standards for approval of flibanserin exceed those required for male erectile dysfunction drugs.

In a follow-up letter two days later, the groups renewed their calls.

"We were pleased, Dr. Woodcock, with your recognition that 24 treatment options for men contrasted with an ongoing dilemma to just approve one for HSDD in women may signal gender bias, conscious or unconscious," they wrote.