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Testosterone can be administered parenterally , but it has more irregular prolonged absorption time and greater activity in muscle in enanthate , undecanoate , or cypionate ester form. These derivatives are hydrolyzed to release free testosterone at the site of injection; absorption rate (and thus injection schedule) varies among different esters, but medical injections are normally done anywhere between semi-weekly to once every 12 weeks. A more frequent schedule may be desirable in order to maintain a more constant level of hormone in the system.  Injectable steroids are typically administered into the muscle, not into the vein, to avoid sudden changes in the amount of the drug in the bloodstream. In addition, because estered testosterone is dissolved in oil, intravenous injection has the potential to cause a dangerous embolism (clot) in the bloodstream.
According to a 2009 paper authored by GTx, "Readers are cautioned to note that the name ostarine is often mistakenly linked to the chemical structure of [S-4], which is also known as andarine . The chemical structure of ostarine has not been publicly disclosed."  While GTx has not formally disclosed the structure of enobosarm, the chemical composition of enobosarm is revealed in patent databases such the WIPO  and discussed by Zhang et al., 2009 in the primary literature.  Various SARM chemotypes exist (aryl propionamides, quinolines , quinolinones, bicyclic hydantoins), though aryl propionamides such as enobosarm, andarine/S-4, and S-23 represent some of the most advanced putative therapeutics under investigation.  In terms of atom connectivity, enobosarm differs from andarine by cyano substitutions on the phenyl rings as it replaces both the nitro and acetamido moieties.