Tamoxifen exerts its therapeutic effect in breast cancer primarily by inhibiting estrogen receptors on breast cancer cells. It acts as a selective estrogen receptor modulator (SERM), binding to estrogen receptors and blocking estrogen from stimulating tumor growth. This inhibition reduces cell proliferation in estrogen receptor-positive breast cancers, thereby slowing disease progression. Tamoxifen’s targeted action makes it effective in both treatment and risk reduction for hormone-responsive breast cancer.

Reference: DeVita, Hellman, and Rosenberg's Cancer: Principles & Practice of Oncology, DeVita et al., 11th Edition.