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Changing the way women are treated for breast cancer could improve their overall chance of survival, a study reveals.

The study shows that switching to a drug called exemestane, two to three years after commencing standard therapy with the drug tamoxifen, can cut the risk of death by a further 17 per cent compared with using tamoxifen alone.

Postmenopausal women with early-stage hormone-sensitive primary breast cancer are usually treated with tamoxifen for five years, once they are free of disease, to reduce the risk of their cancer recurring.

This therapy was once viewed as the 'gold-standard' treatment and it has been shown to cut the risk of death by 34 per cent.

Over recent years, increasing numbers of these women have been receiving treatment with tamoxifen followed by Aromatase Inhibitors such as exemestane.

The study, which involved women from 37 different countries, has been examining the benefits of taking tamoxifen for two to three years and then switching to exemestane for the remainder of the five-year period.

This new research is the first to show that early benefits of the tamoxifen and exemestane treatment sequence are maintained after treatment has stopped.

The study, which was led by researchers from Imperial College London and The Institute of Cancer Research, was funded by Cancer Research UK and Pfizer.

Professor Charles Coombes, Imperial College London: "This study shows that, in order to get best results, patients need to be treated with a sequence of anti-hormonal treatments."

The majority of breast cancer cases are hormone-sensitive, meaning that the cancer cells respond to oestrogen and die when they are deprived of the hormone.

Tamoxifen works by preventing oestrogen from acting on cancer cells, whereas exemestane is an Aromatase Inhibitor, which works by stopping the body's production of oestrogen.

The researchers believe that during treatment with tamoxifen, some cancer cells can become resistant to the effects of the drug.

Exemestane is subsequently able to kill these resistant cells by withdrawing the oestrogen from circulation.

The researchers examined 2,352 postmenopausal women with early-stage breast cancer who switched to exemestane, compared with another group of 2,372 women who were treated with tamoxifen alone.

The women were halfway through their five-year tamoxifen treatment when they joined the study and they were followed up for a median of 56 months after this point.

The study found that the women taking exemestane had a 15 per cent lower risk of dying than those taking only tamoxifen.

When women whose tumours were found not to be hormone sensitive were excluded (8 per cent of the total), the improvement increased to 17 per cent.

The results of the study also suggest that sequential use of tamoxifen and exemestane is safe and well tolerated.

Lead researcher Professor Charles Coombes, from the Cancer Research UK Department of Cancer Medicine at Imperial College London, said: "This study shows that, in order to get best results, patients need to be treated with a sequence of anti-hormonal treatments.

"Just giving one or other drug, such as has been done in some other studies, has not been shown to give added benefit in terms of improved survival.

"The task now is to determine what other drugs should be given in sequence to prevent cancer cells that have become resistant to exemestane from growing."

The study is published in the Lancet.