Hormones through skin patches are directly absorbed into the bloodstream, and therefore can be given in far lower doses to be effective, she explained.

"This study, like others, may change things" in terms of what hormones women and doctors are willing to use, Bradshaw said. Before the Women's Health study, Prempro and Premarin accounted for half of the hormones she prescribed. Now they account for about one-fourth, and much of that is the lower dose of Prempro that Wyeth began selling in 2003, a year after the Women's Health study.

Premarin and Prempro remain top sellers in the United States, according to IMS Health, a drug marketing information firm.

A Wyeth physician, Dr. Eileen Helzner, noted that the French study did not randomly assign some women to get patches and some to get pills — the most rigorous scientific test. At least two previous, smaller studies reached differing conclusions on the clot risk, and more research is needed before definitive conclusions can be made, she said.

She also noted that the federal Food and Drug Administration has not changed its Internet advice that all forms of hormone therapy "have the same risk profile."

However, some women already have moved to patches.

Barbara Isaac, a research nurse and women's health study coordinator at Albert Einstein College of Medicine, switched to Vivelle, sold by Novogyne Pharmaceuticals, after years of taking Prempro and Premarin.

"It's a tiny little patch. It's the size of a small postage stamp. You put it on your belly and change it twice a week," she said.

Although she is 63 and has used hormones for more than a decade, she is not willing to quit.

"I have a very busy life. My sleep is important to me," she said. Before starting on hormones she had "drenching, drenching sweats — you could just wring me out," she said.

"My doctor who prescribed this for me is 70 or 71 and she still uses it," she added.

Isaac and Manson are helping conduct the Kronos Early Estrogen Prevention Study, or KEEPS, which is directly comparing pills and patches.

Ironically, some hormone patches seem to carry greater risks of blood clots than pills when used for birth control. The FDA recently required new warnings on one such contraceptive patch — Ortho-McNeil Pharmaceutical's Ortho Evra.

Also, women should tell their doctors about any other drugs, vitamins or supplements they are taking. The herbal supplement St. John's wort can decrease blood-estrogen levels, undermining the relief any hormone treatment can give. So can certain antibiotics and other drugs.