My Mother-in-Law was on Tamoxifen, as I understood it, Oestrogen is what can stimulate breast cancer cells to divide and grow, but the Tamoxifen prevents the Oestrogen from going in to the breast cancer cells. Or to help prevent it from going into the other breast. If you are more concerned, give the oncologist a call and voice your concern that it would make cancer appear possibly elsewhere, and let them explain the benefits and possibilites of that happening.