system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do medications tend to give us the same common side effects (ie nausea, dry mouth etc.)?
gpt
Your body is connected and built on a foundation of systems which all communicate and respond to each other. Changes in one system tend to cause responses in many others.

Nausea, dry mouth, and other common side effects are usually caused by changes in the sympathetic (fight-or-flight) and parasympathetic (rest-and-digest) nervous systems.  This is important, because many medications influence these two systems either directly or indirectly. 

For example, nausea and dry mouth *usually* come from stimulation of the fight-or-flight system. When your body is "stimulated" this way, you shut down certain functions to focus energy elsewhere. We use drugs like pseudoephedrine for colds, or Adderall for ADHD, and these all activate that system. Beyond that, lots of other drugs will increase fight-or-flight in a roundabout way. Indirectly, any treatment that makes a difference is probably going to have some influence on how "fight-or-flight" or "rest-and-digest" your body is.