Idoxifene is a non-steroidal estrogen antagonist. It is also structurally analogous to tamoxifen. Idoxifene was both active and well tolerated in postmenopausal women with metastatic breast cancer. Idoxifene had similar efficacy and toxicity to tamoxifen in this randomized comparison. (source: Ann Oncol. 2003 Feb;14(2):233-41).