My understanding also is that the drugs effect different people differently, and so finding the right dose of the right specific drug is an exercise of 'finding what works'.
The question is - how should a medical practitioner determine when and by how much to change a patient's medication schedule?
I think you've answered your own question: titration of dose i.e. slow, incremental changes in dose until desired therapeutic response is achieved.
The best way to understand it is terms of pharmacokinetics: if a drug has a linear dose-response curve, then we can anticipate a slightly higher dose will result in a slightly greater therapeutic effect, and a slightly greater increase in side-effects. The point of titrating the dose is to either stop when one reaches a ceiling effect i.e. increasing the dose won't have any improvement in symptoms, because it will only increase the side-effects, which haven't reached a ceiling yet,.
This is essentially true for all psychiatric drugs, and the same rationale applies to all pharmacotherapies.
With regards to ADHD, my understanding is that severity of symptoms does not predict the dose of psychostimulants required. So the psychiatrist will typically hand a patient a bottle of amphetamine, some dosing instructions, and tell them to sort it out themselves over the course of a few weeks. The whole point with the use of psychostimulants for ADHD is that its mediated by dopaminergic hypofrontality. But if one goes overshoots the therapeutic window, then it will result in worse cognitive functioning, etc.
