pharmacodynamic tolerance
a form of drug tolerance in which the chemistry of the brain becomes adjusted to the presence of the drug, which in turn loses its capacity for modifying brain activity. Neurons adapt to continued drug presence by reducing the number or sensitivity of receptors available to the drug (i.e., down-regulation). This cellular-adaptive tolerance is associated with the use of many drugs, including sedative-hypnotics and psychostimulants, and may be followed by withdrawal symptoms when regular doses of the drug are interrupted. Pharmacodynamic tolerance may be contrasted with metabolic tolerance, in which the body reacts to continued presence of the drug by metabolizing it at an increased rate. Both forms of tolerance lead to higher doses of the drug being needed to produce the same effects.