Nearly all drugs of abuse increase dopamine neurotransmission.
This slide shows the increase in brain dopamine (DA) levels (measured in animals) following exposure to various drugs of abuse. All of the drugs depicted in this slide have different mechanisms of action, however they all increase activity in the brain reward pathway by increasing dopamine neurotransmission. It is because drugs activate these brain regions—usually more effectively and for longer periods of time than natural rewards—that they have an inherent risk of being abused.
As a result of scientific research, we know that addiction is a disease that affects both brain and behavior.