Tamoxifen breast cancer drug. Computer model of a molecule of the breast cancer drug tamoxifen. This drug is a selective oestrogen receptor modulator, a class of drug that works by binding to the oestrogen receptor on cancer cells, where it inhibits cell growth. It is also taken as a drug of abuse by male bodybuilders, as it prevents the gynaecomastia (male breast growth) caused by steroid drugs taken to build muscle mass. The molecule's chemical formula is C26.H29.N.O. The atoms are represented as colour-coded spheres in this space-filled model: carbon (blue), hydrogen (green), nitrogen (purple), and oxygen (yellow).