Mostly depends on if you are trying to inject in veins, muscles or just under the skin.
Veins have a lot of places to choose from, which is good because not everyone’s veins are easy to hit in all the places they like go go for veins, like the inside of the arm and the back of the hand.
For muscles, mostly they like the one on your shoulder because it’s easy to get at without you taking off your clothes, but your thighs are also good and are the prefered place if you have to inject yourself.
For under the skin, that’s when they pinch your shoulder skin to make sure they don’t hit the muscle, but if you do it yourself, your tummy is a great place to not hit muscle and you can even keep your pants on, but if you inject often you can switch things up and also pinch your thigh up same as the arm.
If it’s in your butt, that’s a muscle one, and unless you are a small child, you are probably getting haldol. It’s the best place to do intramuscular when you can’t get the person to STAY STILL.