When a person continuously drinks or uses drugs, his body becomes supersaturated with metabolites (the chemicals the body converts the drugs or alcohol into). These metabolites, although removed rapidly from most bodily tissues, may become trapped in the fatty tissues and remain there for years.

When he tries to quit, these drug or alcohol metabolites can be released back into the bloodstream. This can trigger drug or alcohol cravings which are almost impossible to resist. It would be like trying to quit smoking while continuing to smoke 1 or 2 cigarettes per day.

Presence of these metabolites in the blood, even in microscopic amounts, cause the brain to react as if the addict were withdrawing from the drug. Receptor sites in brain cells that have adapted to large amounts of the drug metabolite are now forced to deal with having only a small amount of the drug metabolite available. The brain "requests" the addict to give it more of the drug. This is called drug craving. The only way to end this is to take more drugs or drink more, and the cycle begins all over again.