[milesvp]

>My understanding is that stimulants do exactly the same thing in a person whether they're "cognitively healthy"

I don’t know a lot about stimulants in general, but I know for caffeine in particular this is not true. As an anecdote, I have a friend that caffeine puts to sleep, she just can’t take it. I’ve come to find out (partly from knowing her) that part of why the FDA doesn’t regulate caffeine, is that it has a very wide range of varying effects on different people.

[somestag]

Well, everyone is different biologically and every drug affects everyone differently, I didn't mean that there's no biodiversity between people. What I meant was that these stimulants don't function differently based on whether you're "cognitively healthy." Methylphenidate doesn't do something different in a person with ADHD versus one without, because it doesn't interact with any mechanisms of the illness, and whether or not you have ADHD you're experiencing similar effects on the drug.

Contrast with, say, SSRIs, which might have some effect in a healthy person but you're looking at a different range of effects compared to someone taking it for depression/anxiety/OCD/whatever.