Interestingly enough, Tamoxifen is a drug that was developed in the sixties as a fertility drug! Today it is primarily used to treat pre and post menopausal women whose cancer was estrogen receptor positive. It is what is known as a SERM-selective estrogen receptor modulator. In very simple terms, it blocks the estrogen in the breast. The thinking behind this is that the breast cancer is caused by an over production of estrogen in the breast and blocking it will keep the cancer cells from growing again. After about five years it loses it's effectiveness.
I must tell you that because of the many side effects--blood clots, vision, liver damage, uterine cancer, stroke to name a few-I was hesitant to try this. It seems from what I've read, there is only a small percentage of effectiveness anyway. But, because I know that I can't do a completely vegan diet nor can I purchase only organic food. I'll do it for two years and then that's it.