Hormone replacement therapy may reduce the risk of dementia, a study has found.

Scientists tracked more than 180,000 postmenopausal women in the UK in the largest study of its kind. They found that those who had used HRT were 10% less likely to develop any type of dementia and 16% less likely to be diagnosed with Alzheimer’s, the most common form, compared with those who had never used it.

The greatest reduction was seen in women using HRT who had undergone surgical menopause, with a 26% lower risk of developing any type of dementia compared with non‑users.

Prof Anne-Marie Minihane, the director of the Norwich Institute for Healthy Ageing at the University of East Anglia, who led the study, said: “While HRT has long been prescribed primarily to relieve menopausal symptoms such as hot flushes and night sweats, this work suggests it may also play a role in long‑term cognitive health for some women.”

Women are more likely to develop dementia than men because they live longer. It may also be related to the effects of menopause on brain metabolism and the impact of the main risk gene for Alzheimer’s, Apoe4, being greater on women.

Minihane said: “We wanted to better understand how HRT could prevent dementia and to assess if particular groups of women may respond differently.”

Using data from the UK Biobank, 183,450 postmenopausal women were tracked over an average of 13 years, during which time almost 4,000 cases of dementia were identified. The scientists looked at whether using HRT of any kind for at least one year changed women’s risk of developing dementia.

Women who naturally had lower oestrogen exposure during their lifetime, owing to starting their periods late or early menopause, seemed to benefit more from HRT, with a 16% lower risk of any type of dementia.

Genetics also played a role. The associations between HRT use and dementia were stronger in women who carried the Apoe4 gene variant. “This is really important because Apoe4 carriers are typically considered at higher risk and have fewer established prevention options,” said Minihane.

The findings, published in the journal Alzheimer’s & Dementia, come with the caveat that they are based on observational data rather than being the gold standard randomised-controlled trial, so it is not possible to prove a causal link.

Those using HRT had a higher average level of education than non-users, for instance. Although socioeconomic factors were taken into account in the analysis, it is still possible that differences between the two groups aside from HRT influenced dementia risk.

Dr Susan Kohlhaas, the executive director of research and partnerships at Alzheimer’s Research UK, said: “This study alone isn’t enough to tell us whether HRT should be used to reduce dementia risk, but it provides further evidence that the timing of HRT and a woman’s hormonal history may matter.

“We now need research that can establish whether HRT itself is responsible for these differences in dementia risk and understand which forms of HRT may have an effect.

“Anyone considering starting or stopping HRT should speak to their GP about the potential benefits and risks for them.”