What makes conversations about this very confusing is the fact that neurotransmitters and "chemical imbalances" cannot be seen in isolation. What we are talking about is signalling inside particular structures. Changes in signalling over time effect changes in structure, just like different behaviour does. (actually different behaviour IS change in signalling)

Thus it's no wonder that psychiatric drugs have so many unpredictable effects. Administration of amphetamine might calm you, by exciting a down-regulating part of your brain, that is "structured correctly" (lack of a better word) and simultaneously give you anxiety by exciting the amygdala. It differs from person to person.

It's also no wonder that people and scientists interchangeably exclaim "Behaviour!", "Genes!", "Education!", "Lack of dopamine!" when trying to explain ADHD. It's a mixture of all of those things, but there is definitely a genetic component, which seems to make certain kinds of people way way less apt in controlling their impulses.

Psychiatric drugs don't change the structure of your brain, but, if administered carefully, they lower the bar to change it by behaviour and thoughts, thus being ultimately able to get rid of the medication all together.