Danazol (INN, USAN, BAN, JAN) (brand names Danocrine, Danol, Danazol, Danatrol, Danoval, Cyclomen, many others), also known as 17α-ethynyl-17β-hydroxy-4-androsten-[2,3-d]isoxazole, is a synthetic steroid that is used primarily in the treatment of endometriosis and is marketed widely throughout the world.[1][2][3][4] It is the derivative of ethisterone (17α-ethynyltestosterone) where the 3-ketone is replaced with a 2,3-isoxazole moiety.[5][6] Danazol was approved by the US Food and Drug Administration as the first drug to specifically treat endometriosis in 1971.[5][7] Although effective for endometriosis, its use is limited by its masculinizing side effects.[8] Since their introduction, danazol has largely been replaced by gonadotropin-releasing hormone (GnRH) agonists in the treatment of the condition.[9]