system
Given the following post, respond as if you're commenting on Reddit:
human
eli5: What determines where a drug or the like is injected?

Like, vaccines are injected in an arm or backside, insulin is injected in the abdomen or hip, an epipen is stabbed into the thigh, and some painkillers and other drugs are injected through an IV. I get some sites, like muscle relaxers being injected directly into a spaming muscle, but what other factors come into play?
gpt
There’s no doubt more nuance than this, but a first cut is : things that need to work now ( painkillers, some drugs ) or have a large volume ( fluid resuscitation) or both ( CAT scan contrast) go in veins. Things that can or should work more slowly ( some painkillers, vaccines) go into muscle, so butt or arm. Things that you’re doing frequently to yourself, though, like insulin, that can take effect slowly can most conveniently go in a spot you can see, as well as reach with both hands, and preferably with your pants on- so that really mostly leaves abdomen. 

Epipen is a bit of a special case, in that you want it to work fast, but it’s going to be used outside of a hospital setting by minimally trained people so IV access is not an option. Muscle is faster acting than under skin, so putting it in a big muscle that you or your companions can identify and access is the best call- so use the thigh. You may trade off the dignified ‘keeping your pants on’ part though.