Dopamine. The stimulant medications (like Adderall (dextroamphetamine and racemic amphetamine) and ritalin), which are the best meds for adhd, work primarily by increasing the neurotransmitter Dopamine in a part of the brain called the prefrontal cortex, which controls executive functioning, which lets you plan, shift from task to task, and focus. People with adhd have a shortage of Dopamine in this area.
Still under stiudy. Most medications that work for adhd have the effect of increasing dopamin, a brain neurotransmitter that is closely linked to the reward system. However, details as to why increasing Dopamine activity seems to be so helpful with this disorder remain to be clarified--and it still isn't clear at all that the problem of adhd itself is a result of poor functioning of the dopamin system.