The results of the trial, with 3,500 patients, suggested that the cancer came back in around 40% of the patients who took the medicine for five years, as compared to those who has taken Tamoxifen for two years.
According to the scientists, taking Tamoxifen for five years gives the best chance of survival from breast cancer.
Cancer Research UK Clinical Research director Kate Law said this large Cancer Research UK-funded study is the first to look at the long-term performance of Tamoxifen – considered to be one of the most important drugs in the history of breast cancer treatment.
"Our research is behind many important drugs, including Tamoxifen, that have contributed to more than three quarters of women with breast cancer now surviving their disease for ten years or more," Law said.