Doctors testing arimidex say there is now firm proof that it should replace the gold standard treatment, tamoxifen.
They found that if arimidex was used in place of tamoxifen the disease would be prevented from returning in 1,000 more cases each year.
The results of the 10-year trial show that arimidex, already in use in Britain, works better and has fewer side effects than tamoxifen. Also known as anastrozole, it has been hailed as the biggest step forward in breast cancer treatment since tamoxifen was developed more than 30 years ago.
Tamoxifen is still important in some cases, such as in pre-menopausal women. The latest trial, led by Professor Jack Cuzick at the University of London, showed that around 80 per cent of patients on arimidex were still cancer-free after 10 years, compared to 76 per cent of those given tamoxifen.
Professor Cuzick said: “These results provide further support for using arimidex as the first-line treatment.”
The study, published in the journal Lancet Oncology, looked at 9,000 women in 21 countries, including 3,000 in Britain.