An excessive dose, especially of a narcotic. According the WHO an overdose is "the use of any drug in such an amout that acute adverse physical or mental effects are produced. Deliberate overdose is a common means of suicide and attemped suicide... Overdose may produce transient or lasting effects, or death; the letal dose of a particular drug varies with the individual and the circumstances."
According Schmidtbauer/Scheid the "therapeutic width" of a drug amounts the "distance between the dose, which causes the disired psycho physical condition, and the dose, which the body straight still stand". By the habituation effect (tolerance development) the drug consumer needs however increasing doses, in order to obtain the disired effect. It comes nearly inevitably to collapse, which can lead to death. In addition, after physical withdrawals this tolerance threshold sinks again and straight with back cases therfore deadly overdosing can happen. See also drug death and intoxication.