Another study confirmed that tamoxifen protected women whose family has a history of breast cancer. The 10-year double blind trial of 7,000 women aged 35-70 (the age group considered particularly at risk of breast cancer) found that more of the women on a placebo developed the disease (101) compared to those on tamoxifen (68). This amounted to a 30%-40% reduced risk for high risk women.

The researchers cautioned, however, that tamoxifen could increase the risk of other diseases, such as endometriosis and blood clotting leading to pulmonary embolism. During the trial, for instance, 43 women taking tamoxifen developed a clot compared to 11 on the placebo. They estimated that tamoxifen will prevent 78 breast cancers, but cause one embolism for every 1,000 women treated across 10 years.