The American Cancer Society has said that tamoxifen prevents far more second breast cancers and deaths than it causes. The society was responding to media headlines triggered by a US study showing a rise in numbers of oestrogen receptor negative breast cancers in the contralateral breast in women who had been taking tamoxifen for five years or longer.

The study, conducted by the Fred Hutchinson Cancer Center in Seattle, Washington, reported that women who took tamoxifen for more than five years after treatment for primary oestrogen receptor positive breast cancer had a fourfold higher risk of oestrogen receptor negative breast cancer, which is more difficult to treat, in the opposite breast (Cancer Research 2009;69:6865-70, doi:10.1158/0008-5472.CAN-09-1355).

The observational study …