The way it was explained to me is that individuals with ADHD have a chemical/neurotransmitter imbalance where some are too low, so stimulants raise those levels to balance them out. This is the same reason caffeine tends to have a different effect on people with ADHD. (because it's a stimulant) Also, this is why you shouldn't drink caffeine when you take your medicine, because it can increase the levels too high, which causes them to be imbalanced again. (or if your meds aren't helping, drinking caffeine could help boost them a little, but you have to be careful with this because it can cause problems for your heart)