Psychological addiction, as opposed to physiological addiction, is a person's need to use a drug out of desire for the effects it produces, rather than to relieve withdrawal symptoms. Heroin, for example, produces a physical dependence; the drug eventually takes the place of natural endorphins, so that addicts may use heroin simply to reduce pain. Other drugs, like marijuana, do not create a physical dependency. However, one may become psychologically addicted if he/she comes to depend upon the drug's effect as part of normal existence.
Some doctors make little distinction between the two types of addiction, for the result —substance abuse— is the same. The cause of the addiction in either case is much different, though, as is the type of treatment preferred.