As of 2016, testosterone cypionate is the most popular form of testosterone therapy out there on the Peak Testosterone Forum – significantly ahead of other injectibles, transdermal testosterones and Low Dose Clomid Therapy / HCG Monotherapy. And, in my opinion, it has one of the hightest success rates, assuming good early and followup protocols are implemented. However, a rather controversial debate that goes with the use of cypionate is whether intramuscular or subcutaneous is better. I won’t go into the details of that debate here, because I want to cover a very practical matter instead: where is the best place(s) for a man to self-inject IM (intramuscular) cypionate? And I bring up self-injection, because very few doctor’s offices or TRT clinics require their patients to come in for an injection any more.
There are basically three choices, and I will discuss the pros and cons of each:
a) Quad. The quadricep (large front leg muscle between the knee and groin) is a time-tested location for a man to give himself an IM injection. In fact, I would argue that it is really the only place where you can give yourself a larger volume IM injection. 1 inch needles are typically used for leaner men and up to 1.5 inch for men with quite a bit of leg fat. One of our longtime members explains how he found the best location for giving his shot:
“Hopefully your doctor or his nurse gave you instruction on how to best perform the injection. Basically the three things that minimize the discomfort are:
1) With your opposite hand (from the hand holding the syringe) gently spread the skin a bit taught by expanding the distance between your index finger and thumb.
2) Enter the injection spot pretty close to 90 degrees with respect to the skin surface.
3) It seems best if you quickly sort of “stick” the needle all the way in without slamming it of course.
As far as injection sites I have tried both sides of the glute and I have tried my upper outer right quad (that location is better suited for self injection). So for me none of the injections have been very painful.” 
CAUTION: Always go with recommendations given by your healthcare provider.
“However, when I did the thigh, I thought “hey that didn’t hurt at all!” And then the next day came and I had a helluva charlie horse for about a day and a half. Then I went back to dear wifey for the glute injections.” 
b) Deltoid. Injecting in the delt (shoulder) muscle has become increasingly popular. It also is easily accessible and in many ways ideal for men who are doing more frequent, smaller volume IM injections. One of our forum moderators wrote that he handles his delt injections in the following way:
“I use 1/2″ needles for my injections (mostly quad, delt, and VG) and they work great. BD offers a few 5/8″ options. If I recall they come in 28g, 27g, and 25g. They don’t make smaller gauges in this length because they become a little too flexible. Lately I’ve been using BD 28g 1/2″ insulin syringes. I like them a lot.” 
c) CAUTION: Glute. Some men inject IM into the glute. However, there are a number of risks:
1) To get the right angle, you have to be a 3rd generation yoga master.
2) It is possible to hit the sciatic, an experience you will likely never forget.
The solution around this? Get your partner to inject for you. My wife did this for me for a few months and did a fine job. She hates needles, so I was proud of her!