Shortly thereafter, in 1937, testosterone first became commercially available as a pharmaceutical drug in the form of pellets and then in ester form for intramuscular injection as the relatively short-acting testosterone propionate. Unlike testosterone, AAS that are 17α-alkylated, like metandienone and stanozolol, are orally active. Prominent examples include nandrolone (19-nortestosterone), metandienone (17α-methyl-δ1-testosterone), and stanozolol (a 17α-alkylated derivative of DHT). Another C17β ether prodrug of testosterone, silandrone, also exists but was never marketed, and is notable in that it is orally active. A C17β ether prodrug of testosterone, cloxotestosterone acetate, has also been marketed, although it is little known and is used very rarely or no longer. Major testosterone esters include testosterone cypionate, testosterone enanthate, testosterone propionate, and testosterone undecanoate.
