Drug dependence as pharmacological adaptation

Everybody knows about drug tolerance. The more you drink, the more alcohol you need to get drunk; the more heroin you take, the more drug you need to get high. But what has this got to do with drug dependence? Why should the continued taking of a drug lead to a state where you cannot do without the drug either mentally or physically? The answer probably lies in the way our brains have evolved to control their functioning in the face of alterations in the internal or external environment. The nerve cells which make up the brain are highly capable of adapting to novel situations and the presence of an addictive drug in the brain is just such a novel situation. Faced with an addictive drug, nerve cells gradually compensate for its presence so that they return to relatively normal functioning, even in the presence of the drug. This is what underlies drug tolerance-the ability of the brain to function at higher and higher concentrations of the drug.