Women who take hormone replacement therapy (HRT) to ease menopause symptoms like hot flashes and night sweats may be slightly more likely to develop Alzheimer's disease, a large Finnish study suggests.
Many women have been reluctant to use hormones for menopause symptoms since 2002, when the Women's Health Initiative (WHI) study in the US linked treatments containing man-made versions of the female hormones estrogen and progestin to an increased risk of breast cancer, heart attacks and strokes.
While some previous research has also linked HRT to an increased risk of dementia, results have been mixed and offered little clarity about whether this risk should help inform women's decisions about hormone use.
The current study involved almost 85,000 women diagnosed with Alzheimer's disease in Finland between 1999 and 2013 and a control group of about 85,000 similar women without this diagnosis. Roughly 30 per cent of women in both groups used hormones; most took "systemic" hormones in tablet or pill forms but some used vaginal treatments.
Compared to women who didn't use systemic hormones, those who did were 9 per cent to 17 per cent more likely to develop Alzheimer's disease. The biggest risk was for older women who used HRT for more than a decade.
"These findings should not be a cause for alarm," said senior study author Dr Tomi Mikkola of Helsinki University by email. "For the short-term management of hot flashes, night sweats and disruptive sleep, the benefits of hormone therapy clearly outweigh the risk."
Women typically go through menopause between ages 45 and 55. As the ovaries curb hormone production, women can experience symptoms ranging from irregular periods and vaginal dryness to mood swings and insomnia.
Different types of hormone therapy are available, for example, tablets containing estrogen only or a combination of estrogen and progestogen, as well as transdermal treatments, such as patches, gels and creams.
Alzheimer's disease is the most common cause of dementia among older adults. The progressive brain disorder slowly erodes memory and thinking skills and eventually leaves people unable to handle basic tasks in daily life.
Nearly all of the women in the study who had Alzheimer's were diagnosed at age 60 or older, and 56 per cent of them were over 80 at the time of their diagnosis, researchers report in The BMJ.
Three in four women with Alzheimer's who were taking HRT had been on hormones for more than 10 years when they were diagnosed.
In absolute terms, the researchers calculate, HRT is associated with 9 to 18 additional cases of Alzheimer's disease per year detected in every 10,000 women ages 70 to 80, especially in those who used hormone therapy for over 10 years.
The type of oral HRT - estrogen only or in combination with progestogen - didn't appear to impact the risk of Alzheimer's.
Vaginal forms of hormone therapy didn't appear connected to Alzheimer's disease risk.
The study wasn't a controlled experiment designed to prove whether or how HRT might directly increase risk for Alzheimer's. It also wasn't designed to determine whether certain doses or forms of hormone therapy might directly contribute to that risk.
Evidence from this and other research isn't compelling enough to warn younger women to avoid HRT just because they're concerned about Alzheimer's disease, Dr JoAnn Manson of Harvard Medical School and Brigham and Women's Hospital in Boston writes in an editorial.
"The randomised trials to date support the cognitive safety of estrogen therapy when taken in early menopause," Manson said by email. "For recently menopausal women seeking treatment for bothersome hot flashes or night sweats, these observational findings should not discourage use of hormone therapy or materially influence decision making."