Why do some side effects show up on most medication (headache, nausea, diarrhea, etc.)
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The cause for all of these is pretty wide. They’re all systems with many points of failure and relatively nonspecific symptoms.
Stomach issues like nausea or diarrhea can be related to the medication irritating or disrupting something within the entire GI tract. Of the many processes that happen there, it only has to upset a small number of them in some way for it to have an effect.
Same with headache. Think of all of the different things that cause headaches of various kinds like sound, light, dehydration, alcohol… etc. It’s a relatively non-specific description of “this might make your head feel not great”
Because those are all common ways the body reacts to foreign invaders. Blame your immune system.
Side effect reporting is very conservative. At least in the US, the big list of side effects isn’t the things we’re pretty sure that the medicine caused, or even things that seemed to be more common among people who took the medicine than people who didn’t.
If more than a trivial number of people taking the medicine report headaches, or nausea, or diarrhea, then those get reported as side effects. And those conditions are common enough to get reported for almost any medicine whether or not the medicine causes them.
The trials do report comparative rates of various conditions for treatment and control groups, so physicians can see which conditions the medicine might actually be causing (or at least associated with) and what they might actually want to be careful about. But the big list of side effects in a drug ad goes beyond those conditions.
In really simple (ELI5) terms, medication is seen by your body as an invader. Or it is causing your body to do things it didn’t want or wasn’t ready to do. So the side effects are your body reacting to and/or recovering from this invader or unexpected behavior. Your body doesn’t “know” the medication is trying to help.
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