Women with the most common type of breast cancer could benefit from staying on drugs for a decade, research suggests.
Around 40,000 women have hormone-receptor-positive breast cancer, which is sensitive to oestrogen.
Doubling their drug regime from five to ten years could cut the chance of their cancer returning.
The research was presented at the American Society of Clinical Oncology conference in Chicago and found that women who had aromatose inhibitors (AIs) for ten years rather than five were a third less likely to have their cancer come back.
Lead researcher Paul Goss, professor of medicine at Harvard Medical School, said that the current practice should change so that the drugs are prescribed for longer.
He said: "AIs are now readily available around the world and therefore our results will further improve the outcome of women with breast cancer globally.
"It will help tens of thousands of women. It will have an enormous impact."
Baroness (Delyth) Morgan, chief executive of Breast Cancer Now, called for further research on survival and quality of life, adding: "This a really important study that could one day have a major impact on how we use anti-hormone breast cancer treatments."