Tamoxifen's status as the gold-standard treatment for early breast cancer looked numbered last night as researchers announced promising results for a successor that is more effective but eight times as expensive.

Trials have suggested that anastrozole is superior to tamoxifen, both immediately after surgery and after switching from tamoxifen two to three years after first treatment.

It has performed better than placebo treatments among women who have completed five years on tamoxifen.

The latest study, published in the Lancet medical journal, is based on results among Austrian and German women. It confirms a treatment switch can lower the risk of cancer recurring in the same breast, in the other breast or elsewhere.

But anastrozole, marketed by AstraZeneca as Arimidex, is far costlier, and it will be another 15 months before a decision is made on whether it is supported for widespread use in the NHS.

The National Institute for Clinical Excellence, which decides which drugs will be available on the NHS in England and Wales, is not expected to report on this drug and others in its class until November 2006. Until then, prescribing the drug is a matter for individual doctors, and even then, many NHS trusts will be reluctant to fund it.

Meanwhile, charities are reporting growing pressure from patients understandably eager to learn more about the treatment.

Taking tamoxifen for five years has been the standard drug for the past 20 years for women with cancer that has not spread. Anastrozole has been available for about 10 years, but was mainly targeted at women with advanced breast cancer,.

It has only recently been licensed for early breast cancer in Britain, although it has been used off licence in some cases, including by women unable to take tamoxifen because of side effects. Only one in five newly diagnosed women are on the drug, compared with a third elsewhere in Europe and two-thirds in the US.

Tamoxifen, which cut the chance of cancers recurring by half, has been responsible for impressive improvements in breast cancer survival rates. The Lancet study, which examined two trials of postmenopausal women, compared results for 1,618 women who switched to anastrozole after two years on tamoxifen with those for 1,606 who remained on tamoxifen for another three years.

After two years, those in the anastrozole group had 40% fewer recurrences of cancer or cancer in the other breast: 67 compared with 110. When it came to side-effects, more women on anastrozole had bone fractures but there were fewer with blood clots.

Raimund Jakesz, a professor from Vienna, said: "Although further investigation is necessary to ascertain the ideal sequence and duration of adjuvant endocrine therapy, this analysis confirms that post-menopausal women who receive tamoxifen as adjuvant therapy should be switched to anastrozole after two years of treatment."

Emma Taggart, the policy and campaigns director at Breakthrough Breast Cancer welcomed the news. "Tamoxifen has already made a big impact in reducing the risk of recurrence for certain breast cancer patients, but Arimidex can make a real difference.

"We look forward to further research on how long, and in what order, these drugs should be used ... We hope Nice will take steps to ensure that the drug is available to all those who benefit from it as quickly as possible."

Many breast cancers need supplies of the hormone oestrogen to grow. Studies are under way to compare the effectiveness of drugs such as anastrozole, which block enzymes making oestrogen, both immediately after surgery and after taking tamoxifen.

The government has been under growing pressure from cancer charities to fast-track Nice deliberations on several drugs because of promising trial results. They fear there could otherwise be huge variations across the country over whether local NHS trusts are prepared to fund their use.

Patricia Hewitt, the health secretary, has recently agreed to fast-track the intravenous drug Herceptin, made by Roche and costing up to £20,000 a year, which has shown remarkable results in tackling aggressive breast cancers when used in addition to standard chemotherapy.