Tamoxifen therapy in primary breast cancer: biology, efficacy, and side effects.

Since its introduction into clinical use in the early 1970s, the synthetic antiestrogen tamoxifen citrate has been shown to contribute to the control of human breast cancer in increasingly significant ways. While its mechanisms of action and pharmacology are incompletely understood, tamoxifen appears to act predominantly by blocking the action of estrogen by binding to the estrogen receptor. Clinical trials of tamoxifen for 1 to 2 years in primary breast cancer demonstrate consistent beneficial effects on disease-free survival, but only suggested beneficial effects on survival. Routine use of adjuvant tamoxifen for 5 years or more will depend on the results of ongoing large clinical trials of efficacy and detailed studies of unknown biological effects on other tissues.