Breast cancer drugs that suppress the body's production of oestrogen significantly reduce the risk of premature death, a study has shown.
The findings are expected to influence clinical guidelines on prescribing aromatase inhibitors, which were already known to be effective at preventing cancer recurrence.
Postmenopausal women with the most common type of breast cancer are eligible for treatment with the drugs, which stop tumour growth being fuelled by the hormone oestrogen.
Another widely used drug, tamoxifen, does not halt production of oestrogen but blocks the hormone's ability to bind to molecules on cancer cells.
The new study, which analysed pooled data from nine clinical trials involving 31,920 women, looked at the effect on death rates of taking aromatase inhibitors for five years.
It found that compared with no treatment, the drugs reduced the risk of postmenopausal women with hormone-sensitive (ER-positive) breast cancer dying within 10 years by 40%.
Aromatase inhibitors saved more lives than tamoxifen, which lowered death rates by 30%.
Lead scientist Professor Mitch Dowsett, from the Institute of Cancer Research, London, said: "Aromatase inhibitors remove only the tiny amount of oestrogen that remains in the circulation of women after the menopause - but that's enough to have a substantial impact on a wide range of ER-positive tumours, despite their extraordinary differences at the molecular level.
"But aromatase inhibitor treatment is not free of side-effects, and it's important to ensure that women with significant side-effects are supported to try to continue to take treatment and fully benefit from it."
The research is reported in the latest issue of The Lancet medical journal.
Professor Paul Workman, chief executive of the Institute of Cancer Research, said: "The evidence on aromatase inhibitors has been accumulating for well over a decade, but it has taken this huge and complex study to make sense of all the data, and provide a firm basis for clinical guidelines.
"It tends to be the discovery of new treatments that grabs the headlines, but it is just as important to maximise the benefit patients get from existing treatments, through major, practice-changing studies like this."
An earlier study led by Prof Dowsett showed that women with breast cancer whose tumours were highly sensitive to oestrogen might benefit from extended hormonal therapy, which usually stops after five years.