a constellation of physiologic changes undergone by people or animals who have become physically dependent on a drug or chemical who are abruptly deprived of that substance. The intensity of the syndrome varies with the drug or chemical. Generally, the effects observed are in an opposite direction from those produced by the drug; e.g., the withdrawal syndrome from CNS depressants (e.g., barbiturates) consists of insomnia, restlessness, tremulousness, hallucinations, and, in the extreme, potentially fatal tonic-clonic convulsions. Onset time and severity of the syndrome depend on the rate at which the drug disappears from the body.