The increased risk was seen in both short-term users around the age of menopause (55 or younger), as well as in long-term users. More research is needed to determine if hormone replacement therapy is to blame, the authors wrote, adding that women who require the treatment may be predisposed to dementia for reasons yet unknown.

Researchers examined the medical records of tens of thousands of Danish women who were between ages 50 and 60 as of the year 2000, who didn’t have a history of dementia, and who didn’t have a medical reason not to use hormone replacement therapy.

They found that women who had received estrogen-progestin therapy were at a 24% increased risk of developing dementia and Alzheimer’s disease—even those who began treatment at age 55 or younger.

Rates of dementia were higher among women who had been on the therapy longer. The rate of increased risk ranged from 21% for women who had been on it for a year or less, to nearly 75% for those who had been on it for more than 12 years.

The increased rates were similar among women on daily treatment and women on treatment just 10 to 14 days a month, researchers found. Researchers didn’t find an association between progestin-only therapy or vaginal estrogen therapy and the development of dementia, they noted.

Should menopausal women go off hormone replacement therapy?

The study’s numbers are startling. But its findings shouldn’t influence the decisions of patients and doctors, two U.S. experts—a neuroradiologist from the Mayo Clinic and an epidemiologist at Harvard Medical School—wrote in a corresponding editorial published with the new research.

Prior studies have reported conflicting findings on the risks and benefits of hormone therapy, particularly relating to cognitive functioning and dementia, they wrote.

A 1998 study did not show an increased risk of dementia in women starting hormone therapy between ages 50 and 55, Dr. Stephanie Faubion, director of the Mayo Clinic Center for Women’s Health and the North American Menopause Society, told Fortune.

Two additional studies did not find an increased risk of dementia in women who started hormone replacement therapy shortly after menopause, she added.

Two-thirds of women report “subjective cognitive changes during the menopause transition and may experience a transient slowing of processing speed,” perhaps contributing to the rate of dementia diagnoses seen in the study, she said.

What’s more, it’s not “biologically plausible that hormone therapy used for less than a year causes dementia,” she added.

“No changes should be made in a woman’s hormone therapy based on these findings,” she said, suggesting that future studies examine brain imaging to help pinpoint the effects of hormone therapy, if any, at an early stage.