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 most obvious benefit that injectable steroids have over their oral cousins is that they are not hepatotoxic. Injectable steroids are safer on the liver and the user does not risk hepatic injury because injectable compounds don’t require specific chemical alterations which allow them to survive passing through the liver without being broken almost entirely down. It’s also worth consider that injectable steroids have a much longer half-life due to the ester attached to the hormone. To get the maximum benefit from oral steroids, most must be administered daily, or sometimes they are spread out throughout the day. The ester present in injectable steroids allows a much more infrequent administration and don’t need to be administered daily. More information about Esters