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 this indication.
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.