The Food and Drug Administration has begun proceedings to remove from the market the only estrogen-and-testosterone combination pill for sale in the United States.

The agency has asked the company that makes Estratest to produce evidence that the drug treats hot flashes in menopausal women who don't get relief from estrogen supplements alone.

The FDA approved Estratest in 1976. Research since then has suggested that there isn't good evidence that adding testosterone to estrogen provides better relief for hot flashes, an FDA source said.

In 2000, Estratest was 199th on the list of the top 200 prescription drugs in retail sales. Sales were about $110 million in 2001 and were growing, according to published reports.

Estrogen is the main female hormone. Bloodstream levels of it fall after menopause, causing numerous symptoms that sometimes last months. Testosterone is the main male hormone, though women also make small amounts of it. In both sexes, testosterone is responsible for maintaining libido, or sexual desire.

Sales of pills that combine estrogen and progesterone, another female hormone, have been falling steeply since last summer, when a huge long-term study found that women who took them for post-menopausal "hormone replacement" had a higher risk of heart attack, stroke, blood clots and breast cancer.

Estrogen, alone or in combination with progesterone, is still recommended for treatment of severe menopausal symptoms. At one time estrogen was thought to possibly protect against heart disease, but that was disproved by the large Women's Health Initiative study.

An FDA source said the results of that study were announced after the agency had started examining whether estrogen-testosterone pills were better than estrogen pills alone in treating hot flashes. The FDA's challenge to the claims for Estratest are not a result of the study.

In a statement, the agency said it is also concerned because estrogen-testosterone pills are being prescribed for "female sexual dysfunction" -- loss of libido -- which was not one of the uses approved in 1976.

Karen Carlisle, a spokeswoman for Solvay Pharmaceuticals, the maker of Estratest, said, "We'll exercise our right to provide information and do what the FDA has requested in presenting our position. . . . We will continue to work with the FDA to ensure the continued availability of the product to the hundreds of thousands of women who benefit from it."