The United Kingdom Co-ordinating Committee on Cancer Research said yesterday that although there was 'no guarantee about possible longer-term side-effects', the drug's potential for protecting against breast cancer outweighed possible health risks.
However, the Medical Research Council, which withdrew its support for a large-scale trial in 1992 because of fears the drug could cause liver cancer in rats, said it could not back the trial.
Professor Lewis Smith, director of the MRC's toxicology unit at Leicester University, said: 'We cannot at the moment conclude there is a clear case for the safety of tamoxifen in some of the women who are likely to be in the study. However, I would emphasise that for women at very high risk of the disease (four times the normal risk or greater) the MRC would support its use.'
The UKCCR study is seeking women who have at least twice the normal risk of developing breast cancer. Its secretary, Jean Mossman, said that the committee and Government's Medicines Control Agency was satisfied that the tamoxifen was 'tried and tested'. More than 1 million women world-wide have taken it, 33,000 in clinical trials whose progress had been followed for 10 years or more. Most had few or no side-effects. 'All available data has been reviewed by experts and an independent data monitoring committee set up to review all new data,' Ms Mossman said.
Tamoxifen is a mainstay of breast-cancer treatment and its benefits in women with the disease far outweigh any risks. Its use as a preventive treatment in women who are healthy but have a family history or other risk factors for breast cancer, is more controversial.
Further concerns were raised by a Dutch study published in the Lancet last February which suggested that women taking tamoxifen run up to three times the risk of womb cancer. An unpublished Swedish study suggests that there is an excess risk of colo-rectal cancer although this finding may be due to chance.
The drug's reputation has also been tarnished by an American controversy, political rather than scientific. One leading breast-cancer researcher, Dr Bernard Fisher, is alleged to have withheld information about womb cancer risks, while a Canadian surgeon, Roger Poisson, is alleged to have falsified trial data.
The British study is seeking women aged between 35 and 65 who have a close blood relative with breast cancer. A pilot trial involving about 200 women in four centres is now being extended to include six more in Leicester, Nottingham, Southampton, Cardiff, Birmingham and Glasgow.Reuse content