system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is the difference between medicine injected in the veins through your arms or places with easy access, vs the butt cheeks?
gpt
Medicine injected in the vein enters your bloodstream directly.  This is called *intravenous injection*.

Medicine injected into your body is either *intramuscular* (e.g. the butt cheeks, where it goes into your muscle) or *subcutaneous* (e.g. your ~~arm~~ stomach, where it goes under your skin).

There are other injection sites (e.g. intracavernous where you inject into a sinus) but these three are by far the most common.

Injection method involves four criteria: Effectiveness, safety, patient preference, and economics.

* Effectiveness varies from drug to drug.  Most drugs are most effective when injected intravenously, because that puts the drug directly into your bloodstream with no intermediate effects, but others are most effective when taken up via a muscle.  Subcutaneous is rarely the most effective.
* Safety also varies from drug to drug, but generally speaking intravenous injection is the least safe due to the possibility of vein damage.
* Patient preference is less important, but still a factor.  Intramuscular and subcutaneous injection are preferred by many patients because they can perform it themselves at home with high safety, unlike IV injection which should be performed by a medical professional.
* Finally, economics comes into play.  Some drugs require 10x the dosage when used intramuscularly as opposed to intravenously.  Other drugs require a special formulation for intramuscular injection which is 50 times as expensive as the intravenous formulation.  In these cases, the economics becomes important and IV injection may be preferred despite other drawbacks.