Estrogen pills raise older women's risk of blood clots, but not as much as supplements that also contain progestin, according to new data from a landmark government study on more than 10,000 women.
The study is the most rigorous to date to link estrogen-only pills with blood clots in healthy women past menopause.
While the results were not entirely unexpected, they add to a long list of complications found by the Women's Health Initiative, a large government-sponsored research project that has contradicted long-held assumptions that hormones taken at menopause promote health and postpone age-related ills.
In 2002, the WHI linked estrogen-progestin pills to heart attacks and breast cancer and found that they also doubled the risk of blood clots. As a result, millions of U.S. women stopped taking supplements.
"Most of us didn't anticipate we would be going through this list of negatives when we started 10 years ago," said Dr. J. David Curb, a geriatrics professor at the University of Hawaii and lead author of the estrogen-only study.
Growing list of negatives
"Unfortunately the list just keeps growing," he said. "Almost everything we look at is a negative effect, except for relief of symptoms in women who have severe symptoms."
Women who had previous clots faced the highest risk of developing new clots during the estrogen-only study. As a result, the researchers said estrogen use should be discouraged in such women unless they are taking blood thinners.
Other menopausal women who choose to take hormones should use them only for severe hot flashes and related symptoms, on a short-term basis, they said.
The estrogen-only portion of the WHI, involving Wyeth's Premarin pills, was halted in 2004 when they were linked to strokes. A later analysis also found Premarin increased memory problems.
The newly released data involves a wider variety of blood clots than those that cause strokes.
The research involved 10,739 women ages 50 to 79 with hysterectomies. Those who took estrogen pills were 32 percent more likely to develop blood clots, mostly in the legs, than those on dummy pills. Estrogen-only pills can increase the risk of uterine cancer and are advised only for women without wombs.
Over about seven years, 111 women on estrogen developed blood clots, compared with 86 on dummy pills. The risk was highest in the first two years on hormones.
The study appears in Monday's Archives of Internal Medicine.
Wyeth's Dr. Eileen Helzner said the results confirm information that is already on the pills' label.
Most study women with clots had deep vein thrombosis, which involves clots that form in large veins, usually in the leg. These clots can travel to the lungs, sometimes with deadly consequences.
Dr. Nanette Santoro, a reproductive endocrinologist at Albert Einstein College of Medicine and Montefiore Medical Center in New York, said while the risk for most healthy women on estrogen is relatively low, "it is very important for patients to disclose to doctors if they've had a prior episode of a blood clot." Santoro who was not involved in the research.