The widely-used drug prevents oestrogen fuelling breast cancer in hormone-sensitive patients.
The new trial looked at the effect of taking tamoxifen for a total of 10 years after diagnosis.
The trial, led by Dr Christina Davies from Oxford University, recruited almost 7000 women with oestrogen-sensitive cancer who were completing five years of tamoxifen treatment. Half stopped their treatment immediately while the remainder con-tinued taking the drug for five more years.
The risk of dying during the second decade after diagnosis was further reduced by about one- quarter, leading to a total halving of death rates.