How does tamoxifen actually work?

Blocks estrogen: Some breast cancer cells have receptors on them that bind to estrogen in order to grow. Tamoxifen acts as an "antagonist" and blocks estrogen from binding to these receptors on the cancer cells by binding to them first. With tamoxifen sitting in estrogen's place, the cancer cell can no longer grow.

Receptor blocker: Tamoxifen blocks the estrogen receptor present on er+ breast cancer. It is a serm.