The BBC's Richard Hannaford on the Tamoxifen trials

In April researchers in the US revealed initial results from a study of 13,000 women, which indicated that taking Tamoxifen could prevent breast cancer in 45% of women considered at high risk of getting the disease.

US researchers claimed the drug could prevent breast cancer

But new studies from Italy and the UK, published in the medical journal the Lancet, have cast doubt on the original American findings, and found no firm evidence that Tamoxifen can prevent breast cancer at all. Researchers, however, still think the drug could have a significant role to play.

Breast cancer is the commonest form of the disease in the UK with one-in-twelve women suffering at some point in their lives.

The new research compared women who took Tamoxifen to those who that took a placebo. Both found no evidence that the drug prevented breast cancer.

Serious worries

The BBC's Richard Hannaford explains the Tamoxifen trial

Head of the UK study Dr Trevor Powles, from the Royal Marsden Hospital, London, said he was seriously worried about the thousands of American women now demanding Tamoxifen from their doctors.

Dr Trevor Powles: doctors in the US are under pressure for the drug

Dr Powles said: "There was a huge amount of pressure on doctors following the hype.

"I have very great concerns about the uncontrolled and unmonitored use of Tamoxifen widespread in a healthy population of women."

In Britain where the rules are different women cannot insist on having Tamoxifen as a preventative treatment. Here the drug can only be used to halt the spread of existing cancer and prevent its recurrence.

The British trial was a pilot study set up in 1986 to explore the possibility of testing whether Tamoxifen can prevent as well as treat cancer.

A total of 2,494 women were involved, all with a strong family history of breast cancer, whose progress was monitored for an average of six years.

More than 5,000 women took part in the Italian trial, based at the European Institute of Oncology, Milan. They were followed up for about four years.

Results release too early

Drug could still have a vital role to play

The American researchers were strongly criticised by their British counterparts in April for jeopardising investigations into Tamoxifen by releasing results too early.

Dr Powles said: "My opinion is that the publication was premature. It was very hasty and was not peer reviewed."

It was vital to observe the effects of Tamoxifen over a much longer period before any final conclusions could be reached, said the British researchers.

The women in the American trial were older than in the other trials, and a more wide ranging set of criteria were used to determine their risk of cancer.

Could still work

Despite the latest negative results, doctors are still convinced that Tamoxifen can prevent breast cancer. This is because of the way it blocks the action of oestrogen, the female sex hormone that can make the breast cells of some women go out of control.

Dr Powles said: "Our conclusion at this time is we still believe Tamoxifen will prevent a large number of women from getting breast cancer but we believe it will take at least 10 years to see real prevention occurring."