Now a new analysis of that same research has found the estrogen-progestin combination also appears to increase a woman's risk of stroke, in some instances by as much as 70 percent. The increases were not just limited to women with high blood pressure, which is a known risk factor for stroke.
"We saw an increase in stroke in all postmenopausal women across the board, in healthy women and in women with high blood pressure, in younger women and older women," says study author Sylvia Wassertheil-Smoller, a professor of epidemiology and social medicine at Albert Einstein College of Medicine in New York City. "There is no doubt in my mind that the use of the estrogen/progestin combination should not even be considered as a strategy for protecting a woman's health."
Smoller's research is a re-analysis of data previously released from Women's Health Initiative (WHI), a study designed to examine the health effects of hormone replacement therapy (HRT). Although slated to last eight years, part of the trial ended after five when sufficient data showed combined hormone therapy not only failed to protect women, but also appeared to increase health risks, particularly for cardiovascular disease. The estrogen-only arm of the study is continuing.
The new analysis was devised to see whether the same links appeared in relation to stroke. The researchers looked at data on 8,506 women aged 50 to 79, all of whom received the combined estrogen/progestin therapy. The control group was comprised of 8,102 women of the same age who took a placebo.
The study also took into consideration the women's ages, race, blood pressure status and baseline risk for stroke, none of which were detailed when WHI was halted last year.
The end result: Overall, researchers saw 133 strokes in the group taking the hormones, compared to 93 in the control group. Women aged 50 to 59 who were taking hormones had the most dramatic increases in strokes -- as much as 70 percent over the control group. Those aged 70 to 79 saw the least risk -- only 26 percent.
The most surprising finding of all, Smoller says, was that even in women who had no history of heart or blood vessel disease, stroke rate climbed by 40 percent when the combined hormone therapy was used.
Not everyone agrees with the conclusions, which were presented Feb. 14 at the American Heart Association's International Stroke Conference in Phoenix.
For gynecologist Dr. Steven Goldstein, the new analysis has serious flaws and its conclusions are premature. Among the most obvious problems, he says, is the lack of information concerning other risk factors for stroke, particularly in women with normal blood pressure.
"The study cannot tell us, for example, if these women might have had high cholesterol, if they smoked, if they were overweight -- all factors that could have easily influenced the risk of stroke, irrespective of hormone use," Goldstein says.
He adds that unless researchers can say for certain that these other mitigating factors were not present, then it is impossible to link the incidence of stroke to hormone use in any kind of meaningful way.
"It is very unfair to women to draw conclusions that create fear without sufficient proof that there is even cause for alarm," he says.
While doctors aren't certain how or why the combination of estrogen and progestin appears to increase stroke risks, Smoller believes individual bio-markers and genetic fingerprints may play a role.
According to the American Stroke Association, almost 100,000 women die of stroke each year, almost twice as many as from breast cancer.
To learn more about the other findings from this major study, visit the Women's Health Initiative.
To learn more about risk factors for stroke, check out American Stroke Association.