Discovered in the 1960s, Tamoxifen has transformed the treatment of breast cancer and resulted in nearly 50 per cent fall in death rates in the last 30 years.
But until recently it had been thought that five years of treatment with the drug was enough and longer treatment might carry extra risks.
Now a study of 7,000 women led by the University of Birmingham has confirmed that those who took tamoxifen for a decade had 23 per cent fewer recurrences of their cancer and a 25 per cent lower risk of dying than those who stopped treatment after five years.
The aTTom (adjuvant Tamoxifen - To offer more?) study follows publication of the Atlas trial six months ago by the University of Oxford involving over 12,000 women that reached similar conclusions.
Taken together the results from the two studies, and accumulating evidence worldwide, are likely to change medical practice.
Dr Daniel Rea, clinical lead researcher from the University of Birmingham, said that 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.
As Tamoxifen is cheap and widely available, he said, this could have an immediate impact.
Professor Richard Gray from the University of Oxford presented the results at the American Society for Clinical Oncology meeting in Chicago.