Breast cancer deaths can be halved after 10 years of treatment with the drug tamoxifen, research shows.
The widely used drug prevents oestrogen fuelling breast cancer in hormone-sensitive patients.
Usual current practice is to administer daily tamoxifen for five years. This is known to reduce death rates by around a third during the first 15 years after diagnosis.
The new trial, called Atlas, looked at the effect of taking tamoxifen for a total of 10 years after diagnosis.
Atlas recruited almost 7000 women with oestrogen-sensitive cancer who were completing five years of tamoxifen treatment.
Half stopped their treatment immediately while the remainder continued taking the drug for five more years.
Initially there little difference was seen between the two groups, said the scientists. But after 10 years the additional benefits of longer treatment emerged in women who continued taking tamoxifen.
The risk of dying during the second decade after diagnosis was further reduced by about a quarter, leading to a total halving of death rates.
Most of the extra protection occurred after the end of the 10 year treatment period.
Atlas trial leader Dr Christina Davies, from Oxford University, said: "Around three-quarters of all UK women with breast cancer have hormone sensitive disease, and Atlas shows that 10 years of tamoxifen helps save lives not just during the decade women are taking the drug, but also during the second decade after diagnosis."
Tamoxifen is known to increase the risk of womb cancer in post-menopausal women. Atlas showed that the extra risk of dying from this disease due to tamoxifen was two per 1000 women after five years, and four per 1000 after 10 years.
The trial findings are published in The Lancet medical journal.
Martin Ledwick, from Cancer Research UK, which funded the research, said: "This important study adds further clarity to the question about the length of time women should take tamoxifen. Although treatment for hormone receptor positive breast cancer has become more complex in recent years with some women receiving aromatese inhibitors, these results will help in deciding the length of treatment for women who are prescribed tamoxifen alone."