Just wanted to close the loop on this discussion topic. I had absolutely zero negative effects from having the estrogen injected into my arm. But I do have an interesting observation, based on contrasting how I felt a couple days after the arm injection vs. a couple days after my second injection, which was into the large muscle in my upper thigh.
After the first injection, I had some pretty amazing waves of happiness, almost giddy really for about 3-4 days. That quickly flattened out and by day 12, I didn't feel anything. In fact, I was actually down (of course no way to tell for sure whether that was linked to the medication or all the other stuff in my life).
After the second injection, the rather dramatic "ups" didn't happen. Instead there's been a more steady and somehow deeper feeling of calmness and being just "right".
I am not a doctor and cannot clearly discern why there's been a difference. But I suspect it has to do with location of the shots. Perhaps the shot in the arm moved through my system much faster--probably too fast actually--than the shot in my upper thigh. All I know for sure is that since starting HRT, it feels like a hundred pounds has been lifted off my shoulders