Hi. The efficacy of Indole-3-Carbinol (I3C) supplements as an alternative hormonal treatment for breast cancer has only been SUGGESTED by trials involving very small populations of women (less than 60) and not DEFINITIVELY proven through decades of testing on thousands of women, like Tamoxifen. The safety of this substance in the recommended therapeutic doses of 300-400 mg per day is also not known. There are reports about adverse effects on fertility, and increased incidence of osteoporosis and certain types of cancer with the use of this substance. Until larger, definitive studies are made, I3C cannot be recommended as replacement for Tamoxifen.
If you are post-menopausal, another option is Raloxifene/Evista, which is related to tamoxifen. The side-effect profile is somewhat better than Tamoxifen.