system
Given the following post, respond as if you're commenting on Reddit:
human
How do drugs know where to go in the body?

First of all, I obviously don't think medicine is actually making the choice of where to go.  

But, if I take (for example) acetaminophen for a sprained ankle, adderall for my adhd, and bupropion and lamotrigine for my bipolar, how does all of that get to where it needs to go? Is it just a matter of getting distributed evenly through the body and then absorbed wherever there happen to be receptors that fit the molecules? Doesn't that end up being really inefficient? -- seems like most of the actual meds would just get filtered out pretty quickly.   

Is that were drug interactions come from? -- multiple drugs competing for the same receptors?  

EDIT: holy crap. I was not expecting this quantity or quality of responses. Seriously, i think I learned more here than an entire semester of biology. Every comment is a gem. Thank you all for your time and energy! It is very appreciated.
gpt
You're basically right.  Drugs enter the bloodstream and get distributed throughout, and only "act" where there are receptors that they can bind to (though because shape is everything, there can be some unintentional and/or non-specific binding).  Topical preparations don't have this problem for obvious reasons, but they have to be able to cross the skin, and that's a really effective barrier.

It **is** inefficient and why we sometimes try to deliver the medication as close to the intended/active site as possible and why the pharmacokinetics of a given drug are so important to understand.  It's also why there are sub-therapeutic doses for so many compounds.

Interactions can come from binding to the "wrong" receptors, metabolites of one drug acting on another, slowing the metabolism of another one, or synergies between the drugs.  (and probably other routes).  For example, an antihistamine and alcohol don't bind the same receptors, but they both have the effect of slowing your metabolism and making you tired.  So if/when you take both, you get a synergistic interaction and pass out.