When a person abuses a drug, he develops a physical and psychological dependence on the drug. That is drug addiction. Physical dependence means that the addict's body has become so used to the presence of the drug that if he stops taking the drug, he would suffer painful withdrawal. For instance, he experiences chills, headaches, abdominal pains, nausea, vomiting and muscular pains. Psychological dependence develops when the drug becomes so central to the addict's thoughts, emotions and activities that there is a compulsive craving for it.
Psychological dependence is more difficult to get rid of.
Even after an addict is cured of his physical dependence on a drug, he remains psychologically dependent on it. As drug use increases, the tolerance level develops. This means that larger quantities of a drug are needed to maintain the same 'high'. Soon the addict feels that he cannot survive without the drug as he will suffer from the unbearable pangs of withdrawal. This is when the addict's life turns into a vicious cycle of desperate attempts to get drugs for his next fix to avoid the withdrawal symptoms.