In another unexpected strike against once-popular hormone replacement therapy, researchers have found that the treatment doubles the risk of dementia in post-menopausal women and may even result in mental decline.

Two articles in today’s Journal of the American Medical Assn. add to the growing evidence that so-called combination hormone therapy poses substantial risks and should be used only for temporary relief of menopause symptoms.

“I think it’s safe to say that this is another reason why taking estrogen plus progestin long-term is not a good idea (and) that the risks continue to outweigh the benefits that we see,” said Jennifer Hays, director of the Center for Women’s Health at Baylor College of Medicine and an author of one of the studies.

Early research indicated that continuing hormone therapy after menopause could protect older women against some chronic illnesses. But last July, government scientists halted their landmark Women’s Health Initiative study three years early after finding that the hormones actually increased the risk of heart disease, stroke, blood clots and breast cancer. Millions stopped taking the supplements.

Just as the results of that study surprised researchers, so have the new findings about hormones and mental function.

“We went in with an expectation of benefit and we found double harm,” said Sally A. Shumaker, professor of public health sciences at Wake Forest University School of Medicine and lead author of the dementia study. “The basic bottom-line recommendation is: There is no reason for older women to be taking combination hormone therapy.”

Shumaker said younger women of menopause age may still want to consider hormone supplements, especially if they have particularly severe symptoms — which include hot flashes, irritability and sleeplessness. But, even then, she said, she would recommend the lowest possible dose for the shortest possible time.

Both JAMA articles were based on the same cohort of women ages 65 or older — a subset of those in the Women’s Health Initiative. Participants were randomly assigned to receive a daily dose of combined estrogen-progestin pills or a placebo.

Though the overall chance of developing dementia, including Alzheimer’s disease, was found to be small, the risk doubled for those taking hormones.

After four years, 61 of the 4,532 women in the study had been diagnosed with probable dementia — 40 in the hormone group versus only 21 in the placebo group. That translates into an additional 23 cases of dementia per 10,000 women every year, researchers said.

The reason for the higher risk of dementia among those taking hormones isn’t clear yet. But one possibility, scientists said, is that the treatment increases the risk of miniclots in vessels that circulate blood through the brain.

A second JAMA article found that hormone supplements did not improve overall mental function and, in some cases, even led to mental decline — though not as severe as dementia.

Hays said the only reason for an older woman to continue combination hormone replacement therapy would be to protect against osteoporosis and bone fractures. But there are other ways to prevent bone loss, she added, including getting enough calcium, being physically active and doing weight-bearing exercises.