Addiction can be defined as a state where an organism behaves compulsively, even if the consequences of the behavior do not benefit the organism. Drug addiction is a multi-pronged process. Lifestyle has a role, but the biology of an individual is of prominent importance. The pleasure afforded by various drugs and the craving for these sensations as the effects of a drug begin to dissipate has a physiological basis.

Drugs like morphine, cocaine and heroin work by entering a "reward system" in the brain. As the brain develops a tolerance for the pleasurable sensations evoked by a drug, more of the drug is necessary to elicit those sensations. The term reward system refers to the tendency of humans and other creatures to perform tasks that are rewarding. Pleasurable feelings provide positive reinforcement that encourages repeating of the behavior. Studies in rats have shown that the ventral...