From the Canadian Press:
Breast cancer drug shows better results at preventing return of disease
A newer drug clearly outperforms tamoxifen at preventing breast cancer from returning and should become the first-choice treatment for most women who have had the disease, doctors are reporting.

The drug, Arimidex, might be able to prevent 70 to 80 per cent of the most common type of tumours that occur in women after menopause, compared with the 50 per cent that tamoxifen is credited with warding off, their research suggests.