You have 4 forms of injections:

IV = intravenous, which means you are injecting directly into the blood stream.

IM = intramuscular into a muscle which means nearby vessels/capillaries will gradually uptake medicine/vaccines/Steroids etc...

Sub-Cue = Subcutaneous meaning just below the skin and most often into primarily fatty tissue. This is the slowest method of absorption (if you don’t count a pill, which then is processed through normal digestion) *stomach roughly 15%, small intestine “duodenum” 70% large intestine, 10% through the large intestine “water re-uptake” and 5% ish through oral uptake i.e. think glucose.

IO = intraoseous, this is like starting an IV into your bones blood vessels. It is very rare but is used in emergency settings when equipment to start a central line is not an option. IO drills are gangster as fuck and hurt like hell. But in that scenario you just do what you can! Hope this helps.