Women who take hormone replacement therapy may be at increased risk of breast cancer, heart disease and stroke, a study suggests.

Early results from a major clinical trial in the US indicate that long term use of one type of HRT can seriously damage women's health.

The findings have caused such concern that authorities in the US have ordered researchers to end the study, which involved more than 16,000 women, three years early.

No British women should stop taking HRT on the basis of these results

Professor David Purdie, Hull Royal Infirmary

However, experts in the UK have urged women not to panic, saying the risks of cancer or heart disease remain small.

The study, which was due to continue until 2005, found that women who took a combination of equine hormones oestrogen and progestin were 26% more likely to develop breast cancer.

It also found the hormones increased the risks of heart attacks by 29%, strokes by 41% and doubled the risk of blood clots.

While the treatment reduced cases of hip fractures and colon cancer, authorities ruled that the risks far outweighed the benefits and ordered researchers to stop.

The findings relate to equine or horse HRT and not to natural human oestrogen.

The combination used in the study is not available in Europe, although similar ones are.

The findings will be published in the Journal of the American Medical Association later this month.

High importance

However, researchers agreed to publish the results early because of the importance of the information.

The trial, which was carried out as part of the US Women's Health Initiative, involved 16,608 women between the ages of 50 and 79.

The study found that the increased risk of breast cancer, heart disease, stroke and blood clots applied to all women taking oestrogen plus progestin, irrespective of their age, ethnicity or previous health.

Dr Claude Lenfant, director of the National Heart, Lung and Blood Institute, said the study had been stopped because it was too risky.

"The cardiovascular and cancer risks of oestrogen plus progestin outweigh any benefits and a 26% increase in breast cancer risk is too high a price to pay, even it there were a heart benefit. Similarly, the risks outweigh the benefits of fewer hip fractures."

Dr Garnet Anderson, one of the lead researchers, said: "The trial was stopped at the first clear indication of increased risk."

Medical advice

Dr Jacques Rossouw, acting director of the Women's Health Initiative, advised women to discuss their HRT prescription with their doctor.

"Women with a uterus who are currently taking oestrogen plus progestin should have a serious talk with their doctor to see if they should continue it."

This particular combination is not available here or in Europe, but similar ones are

Malcolm Whitehead, Amarant Trust

The Medicines Control Agency, which licences treatments in the UK, said it hoped to examine the results as soon as possible.

Its figures show that six million HRT prescriptions were issued in England last year.

A spokesman said: "UK product information for HRT already contains extensive warnings about the risk of breast cancer.

"When the data from the WHI study becomes available the MCA will rapidly assess it, seek the independent scientific advice of the Committee on Safety of Medicines and take further action as necessary."

Charities and doctors urged women to discuss any concerns with their GP.

Dr Lesley Walker, of Cancer Research UK, said: "Women need to be aware of the risk of breast cancer and the newly identified risk of cardiovascular disease found in this study when they weigh up the pros and cons of HRT with their doctors."

Malcolm Whitehead, director of the Amarant Trust which provides help and advice to menopausal women, said it was important to remember that not every type of HRT had the same effect.

"This particular combination is not available here or in Europe, but similar ones are, and if you are having long-term treatment you should go to your doctor and talk about the risks and whether you should continue," he said.

Professor David Purdie, from the Centre for Metabolic Disease at Hull Royal Infirmary, urged women not to be frightened by the study.

Speaking to BBC News Online, he said: "No British women should stop taking HRT on the basis of these results and if she is at all concerned she should discuss it with her doctor.

"It is a time for cool heads on this side of the Atlantic."

He said the risks of breast cancer, heart disease and stroke were very low.

A spokeswoman for Breakthrough Breast Cancer said: "It is important for women to have accurate, up-to-date information when weighing up the benefits and risks of taking HRT."