Testosterone injections are huge on The Peak Testosterone Forum when it comes to TRT protocols. Cypionate and enanthate account for about two thirds of all men on TRT according to an ongoing forum poll (as of 9/2015). And, if you throw in Nebido and Aveed undecanoate injections, injections take up about 72% of the total. (This excludes the HCG Monotherapy and Clomid men.) Injections are undoubtedly popular, because they provide (assuming a decent protocol) accurate and youthful testosterone levels. I myself do subQ cypionate injections every other day and really enjoy how I feel, but more on that below.
But are injections safe? Or are there hidden dangers to some men giving themselves testosterone shots? It turns out that that there is a recent study that actually examines that question directly. Furhtermore, it compares injections to two other protocols: patches and topicals. Surprisingly to men like myself enjoying cypionate injections, this study asserts that injections are not as safe as other either patches or topicals. Furthermore, the study was a very large one that pulled in massive patient populations from both the U.S. and the U.K. The authors made the following potentially distrubing conclusions for those of us on testosterones shots: 
--Injections had a 26% higher risk of "cardiovascular events," meaning heart attack, stroke or angina (chest pain) than topicals.
--Injections had a 16% higher risk of hospitalizations than topicals.
--Injections had a 34% higher risk of death than topicals.
So the clear conclusion is that injectible testosterone therapies are more dangerous than other protocols, right? Actually, the answer requires some discussion and is not as obvious as it appears on the surface. And the reason is that it is incredibly common to give "steroid-type" injections on a 2 or 3 week schedule. You read that right: many doctors give MASSIVE cypionate injections to their patients and I have always said that these are a miserable and potentially dangerous experience for the patient.
This was basically the conclusion of a female endocrinologist (Dr. Margarent Weirman), who stated in an accompanying editorial, that "the study raises the issue of whether injectable depo-testosterone or other formulations that consistently result in levels outside the physiologic range should be restricted or at least more carefully monitored for [CV] risk. 
This is a huge admission in my opinion, because Dr. Weirman is an endocrinologist admitting that driving men supraphysiological, i.e. greater than 1200 ng/dl, could be putting men at risk for cardiovascular issues. The reason that this is so significant is that, at least from what I have seen, it is generally endocrinologists who are administering these kind of injection protocols. For reasons that are completely unclear to me to this day, they typically will give a man a massive dose of testosterone cypionate (200-400 mg), which shoots the poor patients testosterone and estradiol through the roof. These protocols are always on an every 2-3 week schedule and about 10 days after the injection the poor guy's testosterone actually crashes through his old levels and for a few days. This means that he will end up with testosterone lower than his old baseline! I document in this page: Testosterone Cypionate - Weekly Versus Every Two Weeks?
Dr. Weirman actually directly states concern about this when she writes that "the outcomes of the analysis support the hypothesis that intermittent pharmacologic levels of testosterone, which are inevitable with an injectable depo-testosterone administration every 2 to 3 weeks, result in an association with an increase in the combined outcome of cardiovascular and cerebrovascular morbidity and death.."  (One caution she adds, though, is that there are other studies that show an increased risk of DVT and expresses surprise.) I can only express thanks for the honest and open assessment of this protocol. It has put countless men on a hormonal roller coaster and just does not take into account basic physiology or cypionate's half life. Again, this study demonstartes that it could easily be causing men actual medical issues.
Some of you may be wondering why this kind of protocol would increase risk for cardiovascular problems. The reason is likely that elevated estradiol increases the risk of clotting and clotting is what strokes and heart attacks are all about. A heart attack begins with a clot in the arteries and a stroke is simply a clot that reaches the brain.
CONCLUSION: In my opinion, injectibles will be exonerated by future research when a frequent and physiological profile is followed, but I certainly cannot prove that. I try to follow a safe philosophy and am currently using a subQ testosterone cypionate protocol myself and inject every other day and my latest total testosterone lab draws were 700 and 800 ng/dl. These are the numbers of a healthy middle-aged male and equivalent to what is reached by many men on the topicals. This helps keep my estradiol at a reasonable level: my last read was 27 pg/ml, i.e. my testosterone and estradiol are within normal youthful ranges.
Did you know you can inexpensively do your own testing for many hormones? The industry leaders is Discounted Labs..
However, I am playing by the rules and not all men on TRT do. Some men (and their physicians) let blood pressure and hemtocrit / hemoglobin go too high. Some men have clotting disorders. And, finally, some men on testosterone deliberately go above 1200 ng/dl, which sends their estradiol through the roof generally. I see this fairly often on The Peak Testosterone Forum, where men are clearly deliberatly injecting way too much testosterone. They are not going to steroid levels, but they are definitely going above 1200 during part of the week.
NOTE: See also my page on Other Potential Risks of Testosterone Therapy.
1) JAMA Intern Med, 2015 Jul 1;175(7):1187-96, "Comparative Safety of Testosterone Dosage Forms"
2) JAMA Intern Med. 2015;175(7):1197-1198, "Risks of Different Testosterone Preparations: Too Much, Too Little, Just Right"