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.
All addictive substances (and many pleasurable activities) release the neurotransmitter dopamine in the nucleus accumbens, a cluster of nerve cells lying deep in the brain. Initially researchers thought that dopamine acted as a hedonic signal — one that registers pleasure in the brain — and that this signal prompted people to continue seeking the substance. But more recent research suggests that dopamine interacts with another neurotransmitter, glutamate, to hijack the brain's system of reward-related learning.