The menopause medication is linked to the deadly disease even when it is used for short periods, according to Danish researchers.
But scientists cannot prove the medicine causes brain damage — women with symptoms bad enough to need it may already be at higher risk.
They studied more than 60,000 women aged over 60 in Denmark from 2000 to 2018.
Dr Pourhadi, of Copenhagen University Hospital, said: “Menopausal hormone therapy was positively associated with development of dementia.
“This was the case even in women who received treatment at the age of 55 years or younger.”
More than 850,000 Brits have dementia, with numbers predicted to rise to more than 1million by 2025.
Women are disproportionately affected by the memory-robbing condition, affecting around twice as many as men.
Around 15 per cent women aged 45 to 64 in England are currently prescribed HRT for symptoms of the menopause.
Previous research has shown long-term use of HRT is linked to a greater risk of dementia.
The latest study, published in the BMJ, also looked at how taking the meds over the short-term could affect the chances of developing the disease.
Of the women included in the study, more than 5,500 were diagnosed with dementia over the 18 years.
Those on HRT for more than 12 years were three quarters more prone on average, with risk rising over twofold in some cases.
This dropped to 21 percent when HRT was prescribed for one year or less.
But despite the strong association, the researchers said they could not prove HRT was causing the link.
Independent experts said the study did not consider other factors that might cause someone to both need the drugs and make them more at risk of dementia.
Dr Susan Kohlhaas, of Alzheimer’s Research UK. said: “The study did not consider other factors known to be involved, such as social isolation, smoking or dietary factors like alcohol.
“Women need to understand the implications of deciding to take HRT in terms of benefits and risks.
“At the moment, as far as dementia is concerned, we’ll need to wait for more research to give clearer answers.
“In the meantime, people should speak to a qualified healthcare professional if they would like to know more about the known benefits and risks of taking HRT.”