Poisoning is usually an acute, short-lived event which necessitates immediate care, though complications such as rhabdomyolysis may persist for a few days. Less commonly, symptoms may arise only after prolonged exposure, as occurs with many heavy metals. Rarely, sequelae may not occur until many years after exposure (e.g. with vinyl chloride). It must be stressed that exposure does not necessarily equate with poisoning as uptake of the agent involved is required but, even if this occurs, poisoning does not necessarily result as the amount absorbed may be too small.
Poisoning may be accidental or deliberate; it is usually accidental in small children, but in adults it is almost invariably deliberate. Less commonly, it may be iatrogenic. Occupational poisoning is frequent in developing countries.