Taking a drug in the years after breast cancer has been successfully treated could lessen the chance of it returning, say researchers.
The drug might prevent new tumours emerging
A worldwide study on letrozole stopped early after researchers said it clearly cut the rate of new tumours.
There is no clear evidence yet that the drug saves lives, and side effects include a higher risk of osteoporosis.
However, Cancer Research UK said the results, published in the New England Journal of Medicine, was "exciting".
Letrozole works by blocking the production of the sex hormone oestrogen, which is known to boost the growth of breast cancers in some women.
It is intended to take over from the drug tamoxifen, which can only be taken for five years before it stops working properly.
The researchers, from hospitals on both sides of the Atlantic, gave the drug to more than 2,500 women, while giving a dummy "placebo" drug to 2,500 others.
Originally researchers had intended to monitor the progress of the women, after five years.
However, after just over two years, they found that rates of recurrence among the group given letrozole had almost halved compared to those in the untreated group.
There was also a marked reduction in the number of women who developed a new tumour in the opposite breast.
This, said the team, justified the decision to stop the trial early and give all the women involved the option of receiving letrozole.
However, the early end to the trial means that the evidence for or against the drug is less than clear-cut, say other experts.
There is no firm proof that, even though there were fewer new tumours in the letrozole group, that women given letrozole were in general less likely to die from breast cancer.
It is possible that women on the drug may still go on to develop the same number of tumours as untreated women - but these simply take longer to arise.
There is some evidence that women taking the drug are at higher risk of brittle bone disease, and there is not yet enough long-term evidence of its side effects, which may also include arthritis and loss of sexual function.
Statisticians Dr John Bryant and Dr Norman Wolmark, also writing in the New England Journal, said that the decision to stop the research "undeniably diminished" the clinical usefulness of the data.
"Although the results demonstrate a meaningful biologic effect of letrozole therapy, they do not demonstrate a significant survival benefit."
They said that "concern" about long-term side effects remained, and that the study, by finishing early, had failed to resolve the question it had been set up to test.
However, in the UK, cancer experts welcomed the results.
Professor Ian Smith, Head of the Breast Unit at the Royal Marsden Hospital in London - one of the centres taking part in the trial, said: "This is one of the most important advances in the treatment of postmenopausal women with breast cancer, and is a further valuable step in preventing disease recurrence."
Professor Jack Cuzick, from Cancer Research UK, said: "Letrozole is a very similar drug to anastrozole, an approved treatment for breast cancer in postmenopausal women which Cancer Research UK is testing as a preventive drug in women at high risk of the disease.
"Both drugs are aromatase inhibitors and work by halting the production of oestrogen - the hormone responsible for the development of many breast cancers.
"The new findings are further evidence that aromatase inhibitors look like becoming the most effective hormone treatments for breast cancer in postmenopausal women, with the potential to save a great many lives."
Doctors are already allowed to use letrozole in the UK for the treatment of advanced breast cancer in some women.
Delyth Morgan, Chief Executive of Breakthrough Breast Cancer, said: "The results of this study look very promising - this drug offers a new way in which disease recurrence could be prevented and increases the arsenal of therapies available for women with breast cancer.
"However, since the trial was terminated early we would welcome more research to establish the long-term effectiveness and side effects of this drug."