Hmm, although I would not mind if my cock was a little bigger, I still like it... and I prefer it healthy and functional. I would definitely advise against any "penis enlargement injections" of any kind - but it is ultimately your decision.
The penis is nothing like a breast - the structures inside it are delicate and sensitive and easily damaged. Hormones, stretching, jelqing
, vacuum pumping are all controversial, but much safer than injections into the penis.
If you are that unhappy with your penis, psychotherapy would probably be a much better course of action.