When an addict initially tries to quit using drugs; cells in the brain, which have become used to large amounts of these drugs, are now forced to deal with much decreased amounts of the drugs. Even as the withdrawal symptoms subside, the brain demands the addict give it more of their particular drug. This is called a drug craving. Cravings are an extremely powerful urge, and can cause a person to create many reasons why they should continue using drugs and or alcohol. He is now trapped in an endless cycle of trying to quit, combined with cravings, relapse, and fear of withdrawal.  When a person drinks or uses drugs over a period of time, the body becomes unable to completely eliminate all of the toxins left behind. These drug metabolites, (the substances the body converts drugs or alcohol into) although removed rapidly from the blood stream, become trapped in the fatty tissues of the body.  This will cause physical and mental cravings, and relapse will remain a cause for concern. Left un-handled, the presence of metabolites even in microscopic amounts cause the brain to react as if the addict had again actually taken the drug, and can set up cravings and relapse, even after years of sobriety.