While doctors continue to unravel the mysteries surrounding addictions, researchers have made considerable inroads in understanding what happens inside the brain when people become addicted. Their effects are seen primarily in the regions of the brain that control the autonomic nervous system, which controls breathing, heartbeat, and other functions over which we have no conscious control. The action on these cells explains physical dependence but not the compulsive behavior associated with addiction. This is controlled by another part of the brain. While some addictive substances bind to the opiate receptors, many highly addictive substances do not.
All addictive substances do affect the brain systems that control motivation.
These actions create and reinforce the compulsion to take the drug.