Women taking hormone replacement therapy (HRT) in the long-term could have a small increased risk of Alzheimer's disease, research suggests.
Experts behind the study say women should be warned of the potential impact of HRT, but UK's Royal College of GPs said the findings should not be a cause for alarm.
HRT is taken to relieve symptoms of the menopause such as hot flushes and night sweats and comes as tablets, gels, cream and patches.
While most experts believe it is a safe treatment, long-term use has been linked to a small increased risk of blood clots, breast and womb cancer.
For the new study, experts examined the use of HRT in 84,739 post-menopausal women in Finland diagnosed with Alzheimer's disease between 1999 and 2013, and compared them with 84,739 post-menopausal women without Alzheimer's.
Almost all of the women with Alzheimer's were diagnosed when they were aged 60 or over. More than half were over 80 when they were diagnosed.
The researchers, led by the University of Helsinki, found that use of HRT was associated with a 9 to 17 per cent increased risk of Alzheimer's disease.
This would mean an extra nine to 18 cases of Alzheimer's disease per year could be expected in 10,000 women between the ages of 70 and 80.
The findings related to long-term use, including women on HRT for over 10 years.
The study found no difference between women taking oestrogen-only HRT or combined oestrogen and progesterone.
Writing in the British Medical Journal (BMJ), the team concluded: "Even though the absolute risk increase for Alzheimer's disease is small, our data should be implemented into information for present and future users of hormone therapy."
"HRT can be of greatest benefit to many women who are suffering from some of the unpleasant side-effects of the menopause," Professor Helen Stokes-Lampard, chairwoman of the Royal College of GPs, said.
"However, as with any medication there are risks and it's important that women are aware of them so that they can make an informed decision, with their doctor, before starting treatment."
