Tamoxifen is given to breast cancer patients to suppress estrogen or both estrogen & progesterone?

Estrogen: Tamoxifen is given to individuals with specific breast cancers that are driven to grow by the presence of estrogen receptors. Tamoxifen interferes with the activity of estrogen in the breast, preventing cancer growth. The role of Progesterone receptors is unclear, though Progesterone receptors in breast cancer are generally associated with estrogen receptors, making tamoxifen an effective therapy.

Estrogen: Tamoxifen is an anti- estrogen in the breast, but can act like estrogen in other organs, notably the uterus. Women on tamoxifen, who have not had a hysterectomy, need to have a gyne keep a close eye on uterine health.

It's complicated: Tamoxifen is a selective estrogen receptor modulator which competes with estrogen to block this receptor on cancer (and other) cells. It does not suppress production of either estrogen or progesterone. Ironically, it is effective against breast cancer cells that harbor either the estrogen or Progesterone receptors (although, tumors that are estrogen rec. (-) and Progesterone rec.(+)are rare.).