Convincing evidence that estrogens can induce breast cancer in women came from studies of the effect of bilateral oophorectomy in women younger than age 35. Such women experienced a 75 percent reduction in breast cancer risk throughout their lifetimes. Additional evidence comes from the observation that the use of estrogen replacement therapy for more than 10 years appears to cause a new breast cancer in one in 100 women. In women with known breast cancer, the antiestrogen tamoxifen reduces the incidence of cancer in the colateral breast by 47 percent over a 10-year period of observation. These data led to the hypothesis that the use of tamoxifen to block the action of estrogen would prevent breast cancer.
Tamoxifen is generally well tolerated and safe.
However, for use in otherwise normal women, infrequent side effects and toxicity become important. Tamoxifen is generally well tolerated, however these conflicting actions must be considered when physicians are estimating risks and benefits in the preventive section. These various actions must be factored in when estimating risks and benefits in the setting of preventing disease in normal women. Breast Cancer Prevention Trials A pilot trial of breast cancer prevention was initiated in 1985 in the United Kingdom with a small number of participants and expanded into a full trial later with the entry of a larger number of women. A large, definitive trial was initiated in 1992 in the United States.