T is commonly administered via intra-muscular injection, so you need a needle long enough to hit your muscle (usually the quadricep, though the gluteous maximus is also used by some people). I do a quadricep (thigh) injection; the nurse who taught me how to inject said that 1.5" 25G is a common size for doing IM injections in the thigh. However, she took a look at my thighs and recommended a 1.25" 27G needle due to my size. She thought that 1.5" would be a bit too long, and she was right (the pharmacy only had 1.5" needles so I started with those but couldn't put them in all the way--I was hitting something hard that I believe was bone).
I purchased some 1.25" and they work much better. However, I couldn't find 25G needles at that length, so I use 27G. It takes a few minutes to inject, but otherwise works fine.
That being said, I have heard of guys who do subcutaneous (in the fat) injections with needles that are similar in length to those used by diabetics. They report T being just as effective when injected in fat vs. muscle, but muscle injections are more common. It is argued that, because T is suspended in a thick oil, it should be injected into tissue that can 'hold' the oil--muscle being more "structurally robust" than fat. Apparently there is an increased risk for abscesses and other injection site complications if T is administered to fat. Some people have been successful with subcutaneous injections; but I've got no problem doing muscle injections with a longer needle, so I'd just as soon not risk it. Talk to your doctor if you would prefer subcutaneous injections.
