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Drug withdrawal is a physiological or behavioural response to reducing or stopping exposure after adaptation to a drug. It differs from simple elimination of the substance: its manifestations reflect the adapted system functioning under changed exposure. It also differs from recurrence of the original condition, although both can occur together. ^[ref]

For example, a system that has compensated for a persistent inhibitory influence may show excessive activity when that influence is removed. The timescale of adaptation can differ from the drug elimination timescale, so a response can continue after concentrations have fallen. Physical dependence, drug tolerance and compulsive use describe different phenomena; the presence of one alone does not establish all the others.

A substance use disorder involves a pattern of substance use associated with impaired control, harmful consequences or disruption of everyday functioning. It can vary in severity and can coexist with other mental disorders.^[ref]

Drug tolerance and Drug withdrawal are physiological concepts that may be relevant, but neither should be used alone as a complete account of the disorder. The pattern of use and its effects on a person's life are separate parts of the assessment.