New York ? Older postmenopausal women who take estrogen significantly increase their risk of developing dementia, whether they take the hormone alone or combined with progestin, according to a study published in today’s Journal of the American Medical Assn.

A related study found that women 65 or older taking estrogen had slightly lower cognitive function, on average, than those taking a placebo — a difference that emerged within a year or two of starting estrogen therapy.

The studies represent the first detailed analyses of cognitive function to include data from the estrogen-only arm of the Women’s Health Initiative, a multiyear clinical trial of more than 27,000 women sponsored by the National Institutes of Health and Wyeth-Ayerst Research.

“It’s clear from the findings that estrogen hormone therapy should not be used to prevent dementia — it won’t prevent it and in fact will increase the risk,” said Stephen Rapp, co-author on both papers and professor of psychiatry at Wake Forest University School of Medicine in Winston-Salem, N.C.

The estrogen-only arm of the study (2,947 women taking estrogen or a placebo) was started in 1995 and halted prematurely in February because of concern about adverse effects, especially an increased incidence of stroke. The estrogen plus progestin arm (4,532 women taking the combination or a placebo) was halted two years ago because of increases in breast cancer, heart attacks, strokes and blood clots.