Most psych meds have GI side effects because your GI system is basically a second brain full if its own neurotransmitters and such, and has like half a dozen interoceptive senses reporting back to your brain only 2 of which you're able to be consciously aware of.
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Hard to control dopamine only in the brain, when half the dopamine in your body is in your gut. https://pmc.ncbi.nlm.nih.gov/articles/PMC10945074/