Dopamine
According to mentalhelp.net, ADHD is related to a lack of dopamine, a chemical the brain produces to control movement and actions. Thus most medications for ADHD work to restore dopamine levels, primarily by stimulating the brain to make more. Dopamine is a neurotransmitter, a chemical brain cells use to communicate with each other. Neurotransmitters are secreted into small areas called synapses, which are spaces in between brain cells where they can interact chemically. Dopamine works in the parts of the brain in charge of "gatekeeping"--the parts that decide which actions and thoughts should be performed or rise to consciousness. If levels of dopamine are low, the brain has a hard time focusing and prioritizing, leading to hyperactivity and problems with paying attention.
Stimulants
Many medications for ADHD are stimulants; examples are methylphenidates, amphetamines, pemolines, and dextroamphetamines. Stimulants treat ADHD by increasing activity in the parts of the brain that help control attention and can also increase the amount of dopamine in the brain. These stimulants bind to and activate specific types of brain cells (called receptors). In response to receptor activation, the brain cells make more dopamine. Common examples of these kinds of ADHD medications include Ritalin, Dexedrine and Adderall.
Nonstimulants
Currently, only one nonstimulant medication, atomoxetine, has been officially approved by the Food and Drug Administration for the treatment of ADHD; but other similar medications are sometimes prescribed by doctors. Atomoxetine decreases the speed at which norepinephrine (another neurotransmitter) is removed from the brain. Norepinephrine, the brain's version of adrenaline, activates the brain similarly to the way stimulants do. Thus the end result of atomoxetine administration is similar to that for stimulants (activation of receptors leading to increased dopamine production), but through a different mechanism--by slowing the disappearance of stimulant molecules rather than by adding more.
