Tamoxifen (Nolvadex) is one of the most commonly prescribed drugs used to prevent breast cancer, either as an initial diagnosis or a recurrence (return). Tamoxifen is a hormone therapy and is known as a SERM (short for selective estrogen-receptor modulator). SERMS, which also include raloxifene (Evista) and Fareston (toremifene) work by blocking estrogen from attaching itself to the special hormone receptors on cancer cells.