tamoxifen

n. medication that interferes with the activity of estrogen in breast tissue, prescription drug used to treat breast cancer (Medicine)

Tamoxifen is an orally active selective estrogen receptor modulator (SERM) which is used in the treatment of breast cancer and is currently the world's largest selling drug for that purpose. Tamoxifen was discovered by ICI Pharmaceuticals (now AstraZeneca) and is sold under the trade names Nolvadex,® Istubal,® and Valodex.® However, the drug, even before its patent expiration, was and still is widely referred to by its generic name "tamoxifen."
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Tamoxifen /ta·mox·i·fen/ a nonsteroidal antiestrogen used as the citrate salt in the prophylaxis and treatment of breast cancer. ta·mox·i·fen n. [more]Tamoxifen - Community and Resources

Noun
1. an antagonist for estrogen that is used in the treatment of breast cancer
(synonym) estrogen antagonist
(hypernym) antagonist

[ta-MOK-si-FEN]
A drug used to treat breast cancer, and to prevent it in women who are at a high risk of developing breast cancer. Tamoxifen blocks the effects of the hormone estrogen in the breast. It belongs to the family of drugs called antiestrogens.