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.
Dave, let me clear the air here on some of the confusion… I recommend the 1 vial cycle for someone who is either A. younger or either B. already has high levels of natural testosterone. Now, the typical middle aged male who already had declining levels of natural testosterone COULD go with 500 mg/wk for 10 wks. Recovery is recovery at that point and if you’re going to do it then I see nothing wrong with getting the most out of that first cycle. But what you have to realize is a male with a starting level of high average test levels can still yield the same benefits of someone who is middle range-low end of normal and uses 500mg/wk of test. In either case the person can still gain a solid 20 lbs of muscle from either 1 or 2 vials, the determining factor on this is what they were at to begin with