Tamoxifen blocks the actions of estrogen, a female hormone. Certain types of breast cancer require estrogen to grow. Tamoxifen is used to treat some types of breast cancer in men and women. Tamoxifen is also used to lower a woman's chance of developing breast cancer if she has a high risk (such as a family history of breast cancer). Tamoxifen may also be used for purposes not listed in this medication guide.