LONDON -- A new breast cancer drug, anastrozole, has been hailed as the biggest step forward in breast cancer treatment for 20 years and may be able to prevent or delay up to 80 per cent of tumors in most high risk post-menopausal women, reports say.

Results of the biggest ever international study on breast cancer showed that the drug was more effective than the current top treatment, tamoxifen, and produced far fewer side effects.

The drug, marketed as Arimidex, hit the headlines earlier this year when news of the trial findings was presented at a conference in Florida.

The full results of the study were recently published in the **** Lancet **** medical journal.

More than 9,000 post-menopausal women from 21 countries who had undergone surgery for breast cancer took part in the study.

The findings showed that three-year disease-free survival was 2 per cent greater among women given anastrozole compared with those treated with tamoxifen.

While tamoxifen reduced the risk of a new tumor appearing in the unaffected breast -- contralateral breast cancer -- by 50 per cent, anastrozole given alone slashed it by up to 80 per cent.

Tamoxifen carries a small but significant risk of causing cancer of the womb lining. This risk was much lower with anastrozole, which also caused less vaginal bleeding, stroke and blood clots, and hot flushes.

However, women taking anastrozole were much more likely to suffer osteoporosis and bone fractures.

The trial was led by Professor Michael Baum, based at the Cancer Research UK and University College London Cancer Trials Center in London. Senior lecturer Joan Houghton, a member of the Trials Center team, said: "The ... trial treated women with existing breast cancer, but we found a reduction in contralateral breast cancer. Therefore there is the suggestion that anastrozole would also be a good drug to use as a preventative agent, especially as side effects are reduced."

Epidemiologist Dr. Jack Cuzick, who analyzed the study data, is now leading a new trial run by Cancer Research UK into cancer prevention. He said: "The results of this study provide great encouragement for evaluating this new drug in post-menopausal women at high risk of developing breast cancer." Professor Gordon McVie, director general of Cancer Research UK, said: "Trials have already shown that tamoxifen can cut down the risk of breast cancer, but there were some concerns about the side effects. Prevention is better than cure and Arimidex is a drug that promises to do better on both prevention and side effects."

Anastrozole is currently only licensed to treat advanced cancer in the United Kingdom. But in light of the trial results, it might now become available for women with early breast cancer by the end of the year.

About 38,000 women are diagnosed as having breast cancer in the UK each year and 13,000 die from the disease, DPA reported.

But the death rate has fallen by 22 per cent in the last 10 years, largely due to better treatment and breast screening.

The lifetime risk of getting breast cancer is one in nine for women.