system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:How do medications know exactly which part of the body to go to?

How does a depression medication know how to go to the brain, an ED pill to go to the nether regions, and vice-versa.
gpt
They don't. They enter the bloodstream and float around until they find a receptor they can act upon. 

The reason why medications seem to only affect the target tissue is because they've been designed to do so. 'Side effects' are caused by the medication affecting tissues that it's not intended to and unbalancing the body in some way. 

Incidentally, ED pills like Viagra were originally designed as blood pressure medication. The ED aspect was actually a side effect.