Hi!
I've heard of several people who were instructed to inject in the upper thigh, but it was painful. My clinic told me to inject in alternate sites across my lower abdomen, where you will carry at least some subcutaneous fat. All I had to do was avoid a 2" area around the previous day's injection site. I've never had any serious trouble with painful injections! It is possible to get a needle that is not as sharp as it should be. I believe I had that occur once, and I've completed 4 rounds, about to begin a 5th round. In later rounds, it seems I will occasionally find spots that are a little tender. I'm assuming these may be past injection sites, since it never happened in the beginning. As long as I move the needle about 1/16th inch over or up or down, the second try is fine.
I do believe Dr Simeons' original directions said to make it a buttocks injection, since I was given HCG once before as a teenager. At that time, it was administered in one side of my upper butt, then the other side next time. The shots were given 3x a week by a nurse, but they did not have a break from P2 after a 6-week period. I think that's why my losses dwindled to nothing and I lost interest.
My recommendation would be to use the abdomen area. That being said, I don't know if any of the areas are "wrong".