I've heard of this concept for ADHD but wasn't sure if it was actually being put into practice. Generally, individuals with ADHD have low dopamine and norepinephrine (and other) levels but may have less of one versus the other--or maybe a lower amount of a different chemical in their brain. There are genetic markers for many of these chemical deficiencies. If the patient is found to have the specific genetic marker signaling low dopamine, for example, then medication that targets dopamine levels--like methylphenidate--should produce a better response.