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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.
The development of gynecomastia or feminization of the breast tissue in males is possible with anabolic steroids. This is due to an excess of estrogen being present in the body, through a process known as "aromatization" whereby androgens like testosterone are converted to estrogen. This excess estrogen then finds its way to the receptors in breast tissue and binds to them. This results in the possibility of female-like breast tissue, which must sometimes be removed by surgery. Most athletes experience itchiness of the nipples, followed by pain. Since this develops over several days, usually, the athlete has more than enough time to discontinue the use of the compounds he´s taking, or to attempt to counteract the breast tissue development while remaining on the cycle. The two most common ways to counteract gynecomastia are the use an anti-estrogen like Nolvadex or Clomiphene Citrate (best taken post-cycle) or Letrozole, a very strong Aromatase Inhibitor (AI)/anti-estrogenic compound is employed during cycle to effectively starve the growth of nourishing estrogen.