Combined hormone replacement therapy has been linked with an increased risk of Alzheimer's disease.
Women take HRT for menopausal symptoms
Women aged 65 and above taking a certain type of HRT had twice the rate of dementia, according to a US study.
The HRT - Prempro TM - is used commonly in the US. According to the Royal College of Obstetricians and Gynaecologists, similar formulations are sometimes prescribed in the UK.
The combination therapy made headlines last year when the Women's Health Initiative found it could increase the risk of heart attack, breast cancer and stroke.
The study was stopped three years early after the US National Institutes of Health ruled the risks of continuing outweighed the benefits.
We recommend that older postmenopausal women not take the combination hormone therapy to prevent dementia
Not all the data had been analysed at the time and scientists have now evaluated its effects on memory.
They are warning doctors not to prescribe combined HRT to older, postmenopausal women to improve memory.
Previous research has suggested that women who take oestrogen-based hormone replacement therapy have a lower long-term risk of developing Alzheimer's disease.
But Dr Sally Shumaker, the national principal investigator of the memory arm of the US study, cautioned against this.
She said: "Because of the potential harm and lack of benefit found, we recommend that older postmenopausal women not take the combination hormone therapy to prevent dementia and we hope that doctors will incorporate what we've learned in their recommendations to women."
She said the Women's Health Initiative Memory Study was the most comprehensive and rigorous to date to test whether oestrogen plus progestin would reduce the risk of dementia in older women.
However, an expert in the UK said the findings were unlikely to apply to many British women.
Peter Bowen-Simpkins of the Royal College of Obstetricians and Gynaecologists said most women in the UK received HRT in their 40s and 50s when their periods were coming to an end.
In the US, he said, it was often prescribed to much older women long after the menopause.
He told BBC News Online: "The message that is coming through from the WHI study is that starting HRT late in life, some years after the menopause may not have the same benefit as taking it when you are still menstruating."
HRT is taken by millions of women around the world to help them cope with the symptoms of the menopause.
Numerous studies are underway to assess its impact on other diseases of later life, such as osteoporosis, heart disease and Alzheimer's disease.