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.
It is very common for bodybuilders to use veterinary steroids for a precontest cycle. Since they are typically assimilated quickly, they do the best work in the shortest amount of time, and are generally out of the system relatively fast in comparison to other ‘roids. And, believe it or not, usually people see fewer side effects when using vet products than when using human ones. Why take anything else? Maybe not ‘why take anything else’, but why not include veterinary steroids, in one form or another, in every cycle? In my mind, veterinary drugs should really be everyone’s choice for extreme condition and definition. They combine well with androgens and other anabolics as well as any drug in the human realm of anabolic steroids. The only problem, these days anyway, is availability. You can find them in Mexico, but you risk fakes, counterfeits, lower quality, or lower dose per ml. You also, of course, face the possibility that you’ll be stopped.
I've been ordering this product from IPS for the past 2 years. They are one of the only sources that carries Testosterone Undecanoate. If you're not familiar with this ester, it's one of the largest one available. I've learned that by adding 250mg/wk of Testosterone Undecanoate to an existing cycle, it keeps my bloods stable which means better gains and overall mood. Also, I can pin Sustanon every 3 days without issue. There is zero PIP with this product. Very light oil which mixes great with a thicker, high concentration Sustanon product I use in conjunction with Testosterone Undecanoate.