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.
Bone Density: An important factor to consider
In general, the maintenance of healthy bone density in all people is partly dependent on both estrogen and testosterone levels. When an individual's body produces estrogen as its main sex hormone (as in the case of female-bodied people), that estrogen in healthy levels protects against bone loss. If a female-bodied person were to begin testosterone therapy, there would be a time of transition in the body while hormone levels adjust. While testosterone would soon become a more dominant presence in the body of a trans man, he would still retain some estrogen in his system, both through the presence of his ovaries (if he has not had an oophorectomy) and/or via the natural aromatization of testosterone into estrogen (which takes place whether or not the ovaries are still present and functional).
The partition coefficient of the ester in question is important because is effects how long the drug itself stays in the system. If the testosterone transfers too quickly from the oil to the blood, the result is a sudden spike in testosterone which then rapidly drops once the dose has been used up. In the example of free testosterone injected into the muscle from a water suspension (as in Aquiviron, mentioned above), the testosterone is essentially immediately available to the bloodstream due to its low partition coefficient, and thus there is an immediate spike of testosterone which is used up quickly in the body.