The new report is the first to include extended follow-up data from the original health initiative study, an additional six to eight years’ worth of information on about 80 percent of the original participants. They took a combination of estrogen and progesterone, estrogen alone or placebos for several years.

For combined hormones, for every 10,000 women taking the drugs, the new analysis found that there were six additional instances of heart problems, nine more strokes, nine more blood clots in the lungs and nine more cases of breast cancer. On the benefit side, there were six fewer cases of colorectal cancer, one fewer case of uterine cancer, six fewer hip fractures and one fewer death. Most of the effects wore off once the drugs were stopped, but the risk of breast cancer remained slightly elevated.

Women who took estrogen alone actually had a reduced risk of breast cancer; the reason is not known. For other conditions, the results were similar to those for combined hormones. But estrogen alone can be given only to women who have had their uterus removed, because estrogen alone increases the risk of uterine cancer. In women who still have a uterus, the estrogen must be combined with some form of progesterone.

For both types of hormone treatments, the risks were lowest in the youngest women, ages 50 to 59, and highest in women from 70 to 79.

In 1993, when the study began, millions of women were taking the drugs to relieve hot flashes and vaginal dryness. There was also a widespread belief that hormones would keep women youthful and feminine, and prevent heart disease and dementia.