Tamoxifen, the breast cancer drug currently in the middle of a controversy over its safety, is to carry a stronger warning in the US. New labelling on the bottle is to point out the increased risks of uterine cancer.

The stronger warning follows the results of a recent Swedish study which showed that the risk of developing uterine cancer was greater than earlier research had indicated. The study, which followed up participants for nine years, reveals that 23 of the 1372 patients randomly selected to take tamoxifen developed uterine cancer compared to four out of 1375 in the control group a risk increased by five times.

A current trial by the National Surgical Adjuvant Breast and Bowel Project in the US is showing similar results after nearly seven years of follow-up.

While the cancer is often diagnosed early, at least six women have died from endometrial cancer after taking the drug.

More bad news about tamoxifen could be on the way in other research, so far unpublished, says Stuart Nightingale, the associate commissioner for health affairs in the US. The new data suggests a possible link to cancers in the gastrointestinal tract. This new data is at variance with previous research, so the results are being re-evaluated.

Nonetheless, the National Cancer Institute in the US emphasizes that the benefits of tamoxifen among women who have already developed breast cancer far outweigh any risks. The NCI is currently carrying out a trial to test tamoxifen as a preventative, although it has been stopped temporarily because the volunteers in the study had not been told of the risks of endometrial cancer. Senators are concerned that the drug may be too dangerous to test or use as prevention. At a senate hearing recently, Cynthia Pearson of the National Women's Health Network said that about two per cent of women who take tamoxifen for five years would suffer either thromboses or uterine cancer.

In other words, for every 8000 women taking the drug, 100 would suffer a major illness and 10 to 15 would die.

The news has also delayed a similar trial in the UK. Researchers are looking for 15,000 volunteers considered at greater risk of developing breast cancer to participate.

The NCI has warned that tamoxifen should not be used as a preventative to breast cancer until the trials are completed and the data published.

!AJAMA, 18 May 1994.