Steroids are chemical compounds or substances that are naturally produced in many plants and animals top play certain biological roles in the body. According to Wikipedia, there are many types of steroids and when categorized by function, four major types are known. These include Estrogens, Androgens, Corticosteroids, and Progestogens. In other words, steroids are basically hormonal in nature. Some steroids are synthetically manufactured and clinically used as prescription drugs in the treatment of certain diseases and medical conditions such as asthma, IBD, eczema, and arthritis, among others.
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.