The findings showed that for every hundred women with breast cancer who completed a full five-year course of tamoxifen, the cancer came back in around six fewer women, compared to those who only took the drug for two years.
"Women diagnosed with early stage breast cancer who are prescribed tamoxifen are recommended to take the drug for five years, but we know that many stop after two or three," said Allan Hackshaw of the Cancer Research UK and University College London Cancer Trials Center, who worked on the study.
"Worryingly our results suggest that by doing this, they could increase their risk of cancer coming back."
Of the nearly 3,500 patients studied in the trial over 10 years, the cancer came back in around 40 percent of those who took tamoxifen for five years, compared to 46 percent among those who took it for two years.
Breast cancer kills around 500,000 people globally every year and is diagnosed in close to 1.3 million people each year. About 75 percent of these cancers are estrogen-receptor positive, meaning they are driven by hormones.
Tamoxifen was the first drug to block the effects of estrogen, and a new generation of drugs known as aromatase inhibitors has now been developed to act in a similar way.