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.
As in all cases you will be best served staying away from Mexican steroids no matter the deal you get. Your best bet, your safest bet and your most effective bet will always be that of human grade steroids. Yes, there are still some fairly decent UGLs in Mexico, perhaps more so than any country in the world but there are so many UGLs there that the majority of these operations are corrupt and dirty. Further, while you still may find quality Mexican steroids, being as they are of the UGL brand understand at any time this quality brand can turn evil overnight making human grade the only thing you should ever consider.
Another common question is whether or not it is legal for someone in the US or UK to travel to another country to buy their steroids. The answer to this question is yes, but it is important to remember that individuals must remain in those countries with those steroids or face felony charges for drug smuggling. For example, people in the United States who want to use steroids legally can (and often do) travel to places like Mexico or Puerto Rico where they can obtain them easily and use them without fear of prosecution. Then, at the end of their cycles, they return home and there is no fear.