International group evaluates tamoxifen risks for women

BMJ 1996; 312 doi: (Published 02 March 1996) Cite this as: BMJ 1996;312:529
  1. Claudia Court

    The drug tamoxifen reduces the risk of contralateral breast cancers, but it also increases the risk of endometrial cancer, according to the findings of an international scientific working group. But the group concludes that no woman being treated for breast cancer should have her treatment stopped on the basis of these conclusions.

    The working group, comprising 17 scientists from a total of eight countries, met at the World Health Organisation's International Agency for Research on Cancer last month to review the evidence on the potential carcinogenicity of a number of pharmaceutical agents. Among the agents considered was tamoxifen, largely because of recent reports indicating a potential hazard in increasing the risk of endometrial cancer.

    The working group reviewed all the published scientific data on second primary tumours reported in patients who had been treated with tamoxifen for breast cancer. It also assessed the evidence for carcinogenic effects of tamoxifen in experimental animals and evaluated possible biological mechanisms of carcinogenesis.

    The group found that “there is conclusive evidence that tamoxifen reduces the risk of contralateral breast cancers.” It also found that “there is sufficient evidence in humans of the carcinogenicity of tamoxifen in increasing the risk of endometrial cancer.” The report also says that there is “inadequate evidence in humans that tamoxifen affects the risk of other cancers.”

    The director of the International Agency for Research on Cancer, Dr Paul Kleihues, said: “It is important to recognise that the findings of the working group do not invalidate the conclusions by clinical oncologists and surgeons that tamoxifen is a very important drug which substantially increases the survival of patients with breast cancer.

    “No woman being treated for breast cancer should have her treatment stopped because of the conclusions of the working group. The risk of endometrial cancer is far lower than the benefits women with breast cancer receive from tamoxifen.”

    However, he adds, “It is important that women have access to scientific opinion on the low risk of endometrial cancer, so that they can make an informed decision on the treatment they will accept.” The National Cancer Institute of America issued a statement in response, saying that the information reviewed by the working group was not new information.

    The institute said that it had previously reviewed the same data and “has already taken this into consideration in the study designs and informed consent procedures in all tamoxifen clinical trials, including the breast cancer prevention trial.” This trial will follow up to 16000 women who have never had breast cancer and who will be taking tamoxifen for five years to see if it prevents the disease—CLAUDIA COURT, BMJ

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