Hair analysis is useful for documenting long-term exposure to drugs. The potential of hair analysis for therapeutic drug monitoring within the forensic field has been studied, but reference values for some antidepressants and antipsychotics in the hair of individuals undergoing chronic therapy are still lacking. In the present study, a method was developed and validated for the determination of 23 analytes, including antidepressants, antipsychotics, and related metabolites, in human hair by liquid chromatography-tandem mass spectrometry (LC-MS/MS). Hair samples (10 mg) were extracted with a 25:25:50 (v/v/v) mixture of methanol/acetonitrile/2 mM ammonium formate (8% acetonitrile, pH 5.3) utilizing cryogenic grinding. The present method demonstrated sufficient selectivity, robustness, and accuracy. Sixteen analytes in hair were reported in 46 psychiatric patients receiving fixed drug dosages. To the best of our knowledge, the hair concentrations of perphenazine and norolanzapine, as well as the concentrations of amisulpride, aripiprazole and its metabolite dehydroaripiprazole, olanzapine, and sulpiride, in hair from individuals receiving fixed dosages is reported for the first time. A significant relationship between the administered dose and the concentration in the proximal hair segment was found only for clozapine, norclozapine, and chlorpromazine. The results confirmed that the idea of using hair concentrations to monitor a daily dose is inapplicable.