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.
Reports of professional athletes who abuse anabolic steroids are increasingly common. Most people know that using steroids is not good for your health, but until now, their effects on the kidneys have not been known. Leal Herlitz, MD (Columbia University Medical Center) and her colleagues recently conducted the first study describing injury to the kidneys following long-term abuse of anabolic steroids. The investigators studied a group of 10 bodybuilders who used steroids for many years and developed protein leakage into the urine and severe reductions in kidney function. Kidney tests revealed that nine of the ten bodybuilders developed a condition called focal segmental glomerulosclerosis, a type of scarring within the kidneys. This disease typically occurs when the kidneys are overworked. The kidney damage in the bodybuilders has similarities to that seen in morbidly obese patients, but appears to be even more severe.