Women who cut short their tamoxifen treatment for breast cancer increase their chances of a recurrence by nearly 6%, a study has revealed.

Data from the Cancer Research UK “Over 50s” trial shows that for every 100 women who received tamoxifen for the recommended five years, 5.8% fewer experienced recurrence, compared with those who received tamoxifen for two years.

Over a ten year period, 3,449 patients aged 50-81 with operable breast cancer, who had been taking 20mg of tamoxifen for two years, were randomly assigned to either stop or continue for an additional three years, if they were alive and recurrence free.

The cancer came back in around 40% of the women who took tamoxifen for five years, compared to 46% among those who took it for two years.

Among women aged 50-59, there was a 35% reduction in cardiovascular events and 59% reduction in death as a result of a cardiovascular event. In older women, the effect was much smaller and not statistically significant.

A statement from the authors of the study, published in the Journal of Clinical Oncology, said: “Women should be encouraged to complete the full course. Although aromatase inhibitors improve disease-free survival, tamoxifen remains a cheap and highly effective alternative, particularly in developing countries.”