Drug addiction, substance dependence or chemical dependency is the compulsive use of psychoactive drugs, to the point where the user has no effective choice but to continue use. The phenomenon of drug addiction and alcoholism has occurred to some degree throughout recorded history, though modern agricultural practices, improvements in access, and advancements in biochemistry have significantly increased the problem in the 20th century.
The addictive nature of drugs varies from substance to substance, and from individual to individual. Drug and alcohol addiction has two components: physical dependency, and psychological dependency. Physical dependency occurs when a drug has been used habitually and the body has become accustomed to its effects. The person must then continue to use the drug in order to feel normal, or its absence will trigger the symptoms of withdrawal. Psychological dependency occurs when a drug has been used habitually and the mind does become emotionally reliant on its effects, either to elicit pleasure or relieve pain, and does not feel capable of functioning without it. Its absence produces intense cravings, which are often brought on or magnified by stress. A dependent person may have either aspects of dependency, but often has both.