HRT has also been connected in recent research to an elevated risk of breast cancer and heart attack, prompting many women to seek herbal or other alternatives to ease menopausal symptoms.
Researchers at the University of Vermont analysed data from 16,608 postmenopausal women and found that HRT containing both oestrogen and progestin doubled the risk of venous thrombosis, a type of blood clot, for all women.
However, the risk was significantly higher still for older women, and those who were overweight or obese, they write in today's issue of the Journal of the American Medical Association (292:1573-1580).
Venous thrombosis occurred in 167 women taking combined HRT, compared with just 76 in the placebo group. Women aged 60 to 69 were 4.3 times more likely to develop a clot if taking combined HRT, while women in their seventies were at 7.5 times the risk.
The risk of a blood clot also increased sharply for overweight and obese women taking combined HRT.
The researchers calculated that over a 10-year period combined HRT would be responsible for an extra 18 cases of venous thrombosis if taken by 1,000 women.
However medical experts now advise that the risks of taking HRT should be weighed against the benefits of the therapy. The American College of Obstetricians and Gynecologists (ACOG) last week drew up new guidelines for use of HRT, noting that herbal supplements do not relieve hot flushes
They also said that about a quarter of US women who stopped taking hormone replacement therapy after it was found to raise the risk of heart disease and some cancers have gone back on it.
But maker of soy isoflavones Solbar said it has seen no decline in sales and other industry members say that ACOG statement should be seen in a positive light.
Joseph Chimbel, president and chief operating officer of herbals company Himalaya USA, told NutraIngredients.com that this revealed that 75 per cent of women leaving HRT chose to stay with herbal or other alternatives.
"This…demonstrates the true success of alternative therapies for menopausal women desiring to avoid HRT and all the proven, negative side-effects associated with this therapy," he wrote in a comment.