Women in the UK with the most common type of breast cancer could benefit from staying on key drugs for a decade, research suggests.
Around 40,000 women in the UK with hormone-receptor-positive breast cancer, which is sensitive to oestrogen, could cut the chance of their cancer coming back if their drug regime is doubled from five to 10 years.
The research, presented at the American Society of Clinical Oncology (ASCO) conference in Chicago, found that women who had aromatase inhibitors (AIs) for 10 years rather than five were a third less likely to have their cancer return or develop cancer in their other breast.
Lead researcher Paul Goss, professor of medicine at Harvard Medical School, said current practice - which is to give women AIs for five years - should now 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."
Prof Goss said the most common recurrence of cancer in the women in the study was "stage four, fatal disease".
He added: "A reduction in recurrences is a very important finding. We have shown that AIs given in a different way than is traditional will remarkably reduce fatal recurrences and recurrences in general and should therefore become the standard practice."
According to Cancer Research UK, women in the UK who have undergone surgery to remove their tumour currently have five years of treatment with AIs.
Some pre-menopausal women also receive prior treatment with another drug, tamoxifen, for three to five years.
Prof Goss said women derived "equal benefit" from staying on AIs for 10 years, regardless of whether they had received tamoxifen beforehand.
"The important thing is that the AIs should be extended beyond five years, to 10 years," he said, adding that women reported no worse quality of life when taking the drugs for longer.
In the study, published in the New England Journal of Medicine (NEJM), 1,918 post-menopausal women with early-stage breast cancer were split into two groups of either five or 10 years on AIs.
While there were differences in how many women had their cancer return, and who therefore needed more treatment, there was no overall difference between the groups in how long women lived.
Analysing the data, Harold Burstein, associate professor at the Dana Farber Cancer Institute at Harvard University and a spokesman for Asco, said women with higher risk breast cancer - equating to about 20,000 women in the UK - would benefit the most from the switch.
He said the findings were important for women, adding: "Nobody wants to have a recurrence.
"'In general, I would think that women who had riskier cancers, higher stage ... would look to these data and think they are compelling for continuing longer durations of treatment."
However, he said women needed to weigh up the benefits compared with the risk of side-effects, which include hot flushes and sweats, nausea, low libido and nausea. Women on AIs are also at higher risk of bone fractures.
In 2013, a study carried out on nearly 3,400 women in Tayside, Scotland, found that only half who had been given tamoxifen or AIs were still taking the pills at the end of five years.
Prof Arnie Purushotham, Cancer Research UK's senior clinical advisor, said the "important study" showed 10 years on AIs could reduce the risk of breast cancer returning.
"It could also help prevent the rare occurrence of cancer being diagnosed in the other breast. We will need more long term studies to see if this could also improve survival for breast cancer patients."
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."