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Long-term use of oral hormone therapy may be associated with a small increased risk of Alzheimer’s disease in postmenopausal women, researchers reported Wednesday.

The Finnish study looked at nearly 85,000 postmenopausal women, between the ages of 70 and 80, diagnosed with Alzheimer’s disease between 1999 and 2013. The researchers found that use of oral hormone therapy for 10 or more years in women who started the pills before age 60 had a nine to 17 percent increased risk for Alzheimer’s disease. Women who used vaginal hormone therapy showed no increased risk.

Prior research has indicated that hormone therapy reduces the risk of vascular dementia.

“It prompted us to do research on Alzheimer’s disease to see if the same results persisted, but it doesn’t look like hormonal therapy provided a protective effect on Alzheimer’s,” said lead author Dr. Tomi Mikkola, supervisor for the obstetrics and gynecology doctoral program in clinical research at the University of Helsinki.

The specific reasons behind this increased risk are elusive, but biological differences between Alzheimer’s and vascular dementia may be one reason why, Mikkola says.

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“Alzheimer’s is a completely different type of disease, we don’t know the mechanism behind the disease. What we know is that the disease has started decades before we see symptoms of memory loss,” said Mikkola.

It is possible that the hormone therapy speeds up progression of the disease, he added.

Alzheimer’s disease is the sixth-leading cause of death in the United States. Nearly 6 million Americans have been diagnosed with Alzheimer’s — two-thirds are women — including 200,000 under the age of 65. By 2050, experts predict that this number will rise to nearly 14 million, according to the Alzheimer’s Association.

“Given the lack of effective Alzheimer’s treatments and increased prevalence of the disease, medical and public health efforts have focused on primary prevention, including risk factors and preventive strategies, especially to women,” said Dr. JoAnn E. Manson, chief of the Division of Preventive Medicine at Brigham and Women's Hospital in Boston, in an editorial written in response to the study.

“But the findings should not be a cause for alarm. For the short-term management of hot flashes, night sweats and disruptive sleep the benefits of hormone therapy seem to outweigh the risk.”

In recent years, considerable attention has been given to the role of menopausal hormone therapy. Two 2017 studies found that the period when a woman starts to produce less estrogen, usually in her 40s, may be a critical point in whether she’ll go on to develop Alzheimer’s or not. Researchers concluded that the hormone estrogen is protective for a woman's brain, stimulating growth and keeping it healthy. But the natural drop in estrogen during menopause means women lose that layer of protection.

Both Mikkola and Manson agree that most women under 60 are safe to use short courses of hormone therapy for menopause symptoms.

“Women should not use hormone therapy for the expressed purpose of trying to improve memory or reduce cognitive decline, but when used for early menopause the benefits are sure to outweigh the risk for short term treatment,” said Manson.

Because the study was observational, it isn't definite that long-term hormone therapy causes Alzheimer's disease. Other risk factors, including cardiovascular disease, diabetes, or having the APOE gene weren't included in the study — these may have also contributed to many of the women's increased dementia risk.

“Women should not be scared to use hormone therapy if needed,” Mikkola told NBC News. “Women who use hormone therapy for symptom relief have a much better quality of life.”

The study was published Wednesday in BMJ.