The drug targets the hormone oestrogen
A breast cancer drug could save 1,300 lives a year if more widely used in the UK, research suggests.
Women with hormone-sensitive breast cancer were either given tamoxifen for a full five years, or switched to exemestane after two to three years.
The Lancet study by UK scientists found the death rate was 17% lower in the group who switched drugs.
Tamoxifen already cuts the chances of dying by 33%, and so overall combining the two drugs halves the risk of dying.
Exemestane, sold under the brand name Aromasin, is in a class of drugs called aromatase inhibitors, a number of which are already available to women in the UK.
An estimated 31,000 post-menopausal women are diagnosed with breast cancer in the UK each year.
In 80%-85% of these cases, the disease is fuelled by the female sex hormone oestrogen.
Tamoxifen interferes with the action of oestrogen, while exemestane reduces its production.
Exemestane is recommended by the National Institute for Health and Clinical Excellence (NICE) as an alternative to tamoxifen after two to three years of initial therapy - but is not available everywhere.
The drug costs around £80 a month, with costs for three years of treatment approaching £3,000.
The charity Cancer Research UK, whose scientists were involved in the study, estimated that using tamoxifen and then exemestane would prevent 1,300 deaths each year if it was rolled out across the UK.
Building on success
Professor Charles Coombes, director of the Cancer Research UK Laboratories and head of cancer medicine at London's Imperial College, London, said: "Tamoxifen has already saved the lives of many breast cancer patients.
"Our latest research shows that we can build on that success by treating women first with tamoxifen then switching to the new drug, exemestane."
Professor Coombes said switching drugs also seemed to avoid the side-effects of long-term tamoxifen therapy, such as cancer of the womb and deep vein thrombosis.
Dr Mary McCormack, consultant clinical oncologist at University College London Hospital, said: "Exemestane is now available in the UK to the thousands of post-menopausal women who have been and will be diagnosed with hormone sensitive breast cancer.
"It is important that women with the disease who are suitable to be treated with exemestane are fully informed of the benefits of switch therapy."
Dr Alexis Willett, of the charity Breakthrough Breast Cancer, said: "This will be very welcome news for many post-menopausal women with oestrogen-sensitive early breast cancer.
"Previous studies had already shown that after switching from tamoxifen, exemestane could reduce the risk of breast cancer returning.
This new evidence shows that it could also improve overall chance of survival.
"Aromatase inhibitors, like exemestane, now increase the treatment options available for breast cancer patients.
"However, it is important to remember that they are not suitable for everyone."