Where does the medicine *go* when you get an injection/shot?

Question

When you get a shot, like a flu or COVID vaccination for example, they typically put it in your upper arm or your thigh. Are they just sort of injecting it into the muscle? Or into a vein? Either way, where does it *go*?
I have seen videos of when people get things like local anesthetic where the skin starts to balloon out where they put the liquid. Does that sort of thing happen just deeper in the body? I know our body is mostly liquid but does it just absorb? Maybe redistributed ?

In: Biology

Answer 1

There are different kinds of injection. The main four types are:

* intramuscular (=into muscle tissue), needle at a 90 degree angle to the skin
* subcutaneous (= under skin but above muscle, aka into fat cells), needle goes in at about a 45 degree angle
* intravenous (=into a vein; arteries aren’t used for this purpose because it would be unsafe), needle goes in at about a 25 degree angle
* intradermal (=within the layers of your skin), needle goes in at roughly a 10-15 degree angle

Different medicines call for different kinds of injections. A vaccine often doesn’t really matter WHERE it ends up, just as long as it gets your body’s attention, so the simple intramuscular injection is perfectly fine. Most medicines or IVs (e.g. blood, glucose solution, saline, etc.) need to be introduced directly into the bloodstream, so those have to be intravenous.

I was a bone marrow donor for someone last year, and for that I self-administered injections of a compound that makes your bone marrow grow, so that it would go into my bloodstream. Those needed to be subcutaneous, as this results in a slower, steadier rate of the drug entering the body and traveling all over.

To answer your last few questions: That “balloon” effect most likely means an intradermal injection, which generally results in very fast absorption by the body, which is useful for pain medication, especially if it’s injected nearby the source of the pain. And yes, the body really just just absorb and redistribute the liquid. Once it’s in your circulatory system, it spreads out, just like if you’d put dye in a collection of glass pipes with flowing water, the dye will spread out until the system is mostly uniformly colored. Depending on where you put the dye, it may spread quickly or slowly–that’s exactly what doctors are trying to make use of when they use different types of injection.