Hormone replacement therapy can protect against heart disease but only if women take it at a particular point in their lives, a study suggests.
Millions of women take HRT
Scientists in the US believe HRT can help to prevent heart attacks if women start taking it before their own body stops producing oestrogen.
They have suggested that taking HRT after this point has no positive impact on the heart and may actually increase the risk of disease.
The study, which is based on a review of other studies, will add to the growing debate over the impact of HRT on women's health.
For years, doctors prescribed HRT in the belief that it protected against heart disease after studies suggested women taking oestrogen had fewer heart attacks.
But recent studies have turned this theory on its head and scientists are now somewhat divided.
A British study, published last September, dismissed claims HRT protected the heart.
Researchers from Manchester University found that women who took oestrogen therapy after a first heart attack were just as likely to have another one as other women.
Two months previously, scientists in the US released early results from a major study involving 16,000 women. They had found evidence to suggest women on HRT were at increased risk of breast cancer, heart disease and stroke.
The question may not be if oestrogen helps, but when is the optimum time to
Professor Thomas Clarkson
The findings prompted US authorities to order scientists to end the study and have led to thousands of women coming off therapy.
But Professor Thomas Clarkson and colleagues at Wake Forest Baptist Medical Center in Winston-Salem, North Carolina, believe that timing is crucial.
Writing in the journal Menopause Medicine, they pointed out that the 16,000 women in the US study had started taking HRT at an average age of 63.
Their review of other studies found that most of those concluding that HRT protects the heart involved women who had started treatment at a much younger age.
They also referred to tests involving monkeys, which have shown that HRT can reduce the build-up of fatty acids in the arteries if taken before the natural supply of oestrogen runs out.
They suggested that this backs up the theory that HRT might help to keep blood vessels healthy when started in younger women with relatively clear arteries. But if given to women with more advanced blood vessel disease, HRT might be
either ineffective or potentially harmful.
Professor Clarkson said: "Mounting evidence points to the conclusion that HRT can help prevent heart vessel disease - if the therapy begins around the time that the body stops making its own oestrogen.
"The question may not be if oestrogen helps, but when is the optimum time to