The UK study found that taking tamoxifen for twice as long as currently recommended drastically reduces the risk of women dying from the most common kind of breast cancer.
Experts said doctors were now likely to change their advice to women, following the research, which was presented at a conference in Chicago.
Around 50,000 women are diagnosed with breast cancer each year in the UK, including 4,500 in Scotland.
A five-year course of tamoxifen is the standard treatment for those whose cancer is known as oestrogen receptor positive, which accounts for around 75 per cent of all breast cancers.
But the new study found that oestrogen receptor positive breast cancer patients taking tamoxifen for longer than the recommended five years were better protected against the disease returning and less likely to die from the disease.
The study, involving researchers at Birmingham and Oxford universities, looked at almost 7,000 women with breast cancer who, after five years of taking tamoxifen, either continued taking the drug for another five years or stopped treatment.
Among those who took tamoxifen for ten years, 25 per cent fewer had recurrences of breast cancer and 23 per cent fewer died compared to women who took it for just five years.
Dr Daniel Rea, clinical lead researcher at Birmingham, said: “These results are important as they establish that giving tamoxifen for longer than the current standard of five years significantly cuts the risk of breast cancer returning.
“Doctors are now likely to recommend continuing tamoxifen for an extra five years and this will result in many fewer breast cancer recurrences and breast cancer deaths worldwide.
“Tamoxifen is cheap and widely available so this could have an immediate impact.”
The female sex hormone oestrogen encourages breast cancers to grow by activating oestrogen receptors. Tamoxifen blocks these receptors, reducing the chance of breast cancer returning after surgery or developing in the other breast.
But despite the benefits of tamoxifen, it can have side effects. Women may experience symptoms similar to those during menopause, such as night sweats and hot flushes.
Rare but serious side effects include increased risk of blood clots, stroke and endometrial cancer. However, in the new study, unveiled at the American Society of Clinical Oncology meeting, no increase in the incidence of stroke was observed after ten years of tamoxifen therapy.
Professor Richard Gray, based at Oxford University, said: “The study establishes that the benefits of taking tamoxifen for longer greatly outweigh the risks.”