Metabolism of antipsychotic drugs usually starts with oxidation by hepatic microsomal enzymes (P450 system), followed by glucuronidation and excretion in urine. After long-term use, the rate of conversion of parent drug increases slightly, causing a mild metabolic tolerance; however, monitoring the blood concentration of drug is generally not useful in preventing this problem. In individual patients, very wide variations in blood concentration of antipsychotic agent can still achieve control of symptoms. Thioridazine because of its prominent anticholinergic activity in the gastrointestinal tract, may display erratic absorption after oral administration. Even with regular dosing, especially in older patients, periods of inadequate or excessive blood concentrations of drug may result.