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.
In addition to the endogenous steroid metabolites highlighted in Figure 15-1 , there are also a variety of drugs with androgenic activity. These include anabolic steroids, such as nonaromatizable oxandrolone , that bind and activate AR (albeit with lower affinity than testosterone [ 43 ]) and a class of drugs under extensive development referred to as selective AR modulators (SARMs), which demonstrate tissue-specific agonist or antagonist activities with respect to AR transactivation [ 44 ]. These orally active nonsteroidal, nonaromatizable SARMS are being developed to target androgen action in bone, muscle, and fat and to influence libido but to not exacerbate prostate growth, hirsutism, and acne. Several have been identified with beneficial effects on bone mass [ 45–47 ] and provide a new alternative to androgen replacement therapy.