In Brief: The Quirky Brain: Why addiction causes craving
When people who are addicted to alcohol, nicotine, or a drug try to kick the habit, the first challenge is dealing with withdrawal symptoms such as agitation, nausea, and problems concentrating. Depending on the substance, the withdrawal process may take days to weeks. But intense cravings for the desired substance may continue for years — often triggering relapse.
Cravings may occur at the sight, smell, or memory of anything associated with the addiction: seeing a hypodermic needle, smelling cigarette smoke, or even walking past a bar. The source of these cravings lies in the brain. The process of addiction not only alters neural circuits involved in pleasure and motivation, but also exerts a lasting hold on people through the process of conditioned learning and long-term memory.