Drug addiction path begins with the act of taking drugs. In time, a person's ability to choose not taking drugs is compromised. This in large part is a result of the effects of prolonged drug use on brain functioning, and thus on behavior. Addiction, therefore, is characterized by compulsive, drug craving, seeking, and use that persists even in the face of negative consequences.

The phenomenon of drug addiction has occurred to some degree throughout recorded history , though modern agricultural practices, improvements in access to drugs, and advancements in biochemistry have exacerbated the problem significantly in the 20th century with the introduction of purified forms of active biological agents, and with the synthesis of hitherto unknown substances, such as methamphetamine. While "addiction" has been replaced by "dependency" as a clinical term, the terms are used interchangeably here.
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.