The current theory suggests that ADHD is caused by a mild disorder somewhere between when dopamine is released to communicate between the brain's neurons via synapses and when most of it is reuptaken back into the brain's dopamine bank and the rest is transported to other parts of the body. Some of this dopamine is converted in norepinephrine (adrenaline).

Explained in the simplest way, ADHD is caused either when not enough dopamine is let out, too much dopamine is reuptaken back into the storage and transported to become norepinephrine, and/or norepinephrine is reuptaken too fast.

This is why, theoretically, different ADHD medications are only effective on different types of ADHD patients.