Many addictive substances increase levels of a chemical in the brain called dopamine. Circuits in the reward system use dopamine to “teach” the brain to repeat actions we find pleasurable—a process called reinforcement. When people take drugs, the brain releases a lot of dopamine, which strongly reinforces the action of taking the drug. Over time, dopamine is released less by taking the drug itself than by other activities, people, and places that are associated with drug use. This leads people to feel highly motivated to take the drug whenever they encounter those cues.