Taking hormonal drugs for up to 15 years reduces the risk of breast cancers coming back, a landmark study suggests.
The trial, involving 1,918 patients, which had top billing at the world's largest cancer conference, showed the risk was cut by a third.
Experts described it as a "big deal" that will change treatment for millions of women.
But they warned there were risks, including osteoporosis.
Globally, 1.7 million women are diagnosed with breast cancer around the world each year.
Around 80% of the tumours are fuelled by the female sex hormone, oestrogen.
Such cancers have a low but persistent risk of returning that lasts for years.
It is why women already take drugs such as tamoxifen, to prevent oestrogen getting into breast cells, or aromatase inhibitors, which stop the body making oestrogen, for years after the lump is removed.
The trial, carried out on post-menopausal women, doubled aromatase inhibitor treatment from five to 10 years.
The data, presented to the American Society of Clinical Oncology (ASCO), showed that cancer recurrence was cut by 34%.
But many women on the trial had already taken other hormonal drugs before starting on aromatase inhibitors and benefited from 15 years of treatment.
Prof Paul Goss, one of the researchers from Massachusetts General Hospital, said: "[The study] will have an enormous impact, a reduction in recurrences is a very important finding.
"Aromatase inhibitors are now readily available around the world and therefore our results will further improve the outcome of women with breast cancer globally."
At the end of the study, 95% of women were still cancer-free if they had taken the extra medication, compared with 91% without.
The study did not show an improvement in survival rates, as patients had not been followed for long enough, but scientists expect this to come as "night follows day".
The results, which have also been published in the New England Journal of Medicine, have been widely praised as significant.
Dr Nick Turner, a breast cancer specialist from the Institute of Cancer Research in London, told the BBC News website: "It is a big deal, it's going to be a change of treatment for a lot women.
"Extended letrozole [an aromatase inhibitor] in years 10-15 has benefit in preventing a new breast cancer diagnosis.
"But this won't be for everyone, many will be low risk and can probably safely stop at five years [of aromatase inhibitors], but then we're talking about a substantial number of women keeping going from five to 10 years [of aromatase inhibitors]."
There were side effects to treatment including loss of libido, hot flushes and vaginal dryness.
The treatment also increased the risk of osteoporosis and bone fractures.
Experts said it should be a decision between doctor and patient whether to continue.
Dr Harold Burstein, from ASCO and the Dana Farber Cancer Institute, said: "I think you can say fairly that for millions of women around the world these data will support longer durations of anti-oestrogen therapy."
But he said the balance of risks and benefits meant the drugs would likely be targeted at those whose tumours were most likely to come back.
He said: "In general, I would imagine that women who had riskier cancers will look to these data and think they are compelling for continuing on longer durations of treatment out to 10 or 15 years.
"But we're certainly not at the point of saying women should be on these drugs for the rest of their lives."
In the UK, more than 40,000 women are diagnosed with an oestrogen-positive breast cancer each year.
Up to three years of tamoxifen, followed by five years of aromatase inhibitors is a common practice.
Dame Valerie Beral, professor of epidemiology at the University of Oxford, told the BBC: "Women who have had hormone sensitive breast cancer can still continue to have recurrences of the disease five, 10, and even 15 years after the cancer was first diagnosed.
"The new trial adds further evidence to support the view that women with hormone sensitive breast cancer should continue to take hormone blocking drugs for at least 10 years, or as long as is feasible."
Baroness Delyth Morgan, the chief executive at the charity Breast Cancer Now, said: "This is a really important study that could one day have a major impact on how we use anti-hormone breast cancer treatments."
Prof Arnie Purushotham, from Cancer Research UK, said it was an "important" finding but called for more long-term studies.
Follow James on Twitter.