Researchers from the University of Oxford have found that breast cancer patients who take the drug tamoxifen are more likely to survive for at least a decade.
The study, published online in The Lancet medical journal, involved analysing trial data of over 20,000 breast cancer patients, who had their long-term progress monitored.
Researchers found that women treated with an early course of tamoxifen were a third less likely to die of breast cancer over a 15-year period. The drug was also found to "substantially reduce" mortality rate well beyond a decade.
The study also showed that tamoxifen reduced the likelihood of the cancer recurring.
Tamoxifen treats hormone sensitive (ER-positive) cancers by attaching itself to molecular receptors on the tumour to prevent oestrogen stimulating growth.
Dr Christina Davies, who took part in the study, said: "This study now shows that tamoxifen produces really long-term protection. For ER-positive disease, tamoxifen reduces 15-year breast cancer mortality by at least a third, whether or not chemotherapy has been given."