Drug addiction is the obsessive use of psychoactive drugs, to the spot where the user has no useful choice but to keep on use. The occurrence of drug addiction has occurred to some degree right through recorded history, though recent agricultural practices, improvements in right of entry to drugs, and advancements in biochemistry have exacerbated the problem significantly in the 20th century with the introduction of purified forms of energetic biological agents, and with the mixture of hitherto unidentified substances, such as methamphetamine. While "addiction" has been modified by "dependency" as a scientific term, the conditions are used interchangeably here.

The addictive scenery of drugs varies from substance to substance, and starting individual to individual. Drugs such as codeine or alcohol, for instance, characteristically require many other exposures to addict their users than drugs such as heroin or cocaine. Similarly, a person who is psychologically or hereditarily predisposed to addiction is much other likely to become reliant.

Drug addiction has two mechanisms: physical dependence, and psychological dependence. Physical dependence occurs when a drug has been used usually and the body has become familiar to its effects. The human being must then carry on to use the drug in order to feel usual, or its lack will trigger the symptoms of removal. Psychological dependence occurs when a drug has been used usually and the mind has become sensitively reliant on its effects, also to obtain pleasure or relieve pain, and does not sense capable of implementation without it. Its lack produces intense cravings, which are frequently brought on or exaggerated by stress. A dependent human being may have either aspects of dependence, but often has equally.

"Chipping" is also a phrase used to explain a pattern of drug utilize in which the user is not physically dependent and sustains 'forbidden use' of a drug. This is completed by avoiding influences that support dependence, such that the drug is used for recreation and not for flees.