Objective To investigate the effect of various comedications on the plasma concentration of clozapine(CLZ) in inpatients with psychosis disorder. Methods High performance liquid chromatography (HPLC) was used to analyze serum CLZ and norclozapine(N-CLZ) concentrations in 571 psychosis patients. The influence of commedication on plasma CLZ concentration was analyzed using SPSS 17.0 software while considering the dosage of CLZ and the comedications with 12 drugs most often used in combination with CLZ. Results The CLZ dosage, plasma CLZ concentration and plasma N-CLZ concentration in the comedication group were significantly lower than those in pure CLZ group (P<0.01). The CLZ dosage in groups of comedications with clonazepam, paroxetine, venlafaxine, aripiprazole, quetiapine and risperidone were significantly lower than that of pure CLZ group (P<0.05,P<0.01). Comedication with clonazepam, lithium carbonate, valproate and quetiapine significantly decreased the plasma concentration of CLZ (P<0.05,P<0.01). Comedication with clonazepam, lithium carbonate, valproate, sertraline, quetiapine and risperidone significantly decreased the plasma concentration of N-CLZ (P<0.05,P<0.01). Spearman’s correlation analysis found that there were significant positive correlations among any two of dosage CLZ, concentration of CLZ and concentration of N-CLZ in CLZ group (P<0.05,P<0.01). Conclusion Some commonly-used antipsychotics have impact on plasma CLZ concentration. Drug monitoring and dosage regulation of CLZ should be stressed among patients comedicated with clozapine combined with other antipsychotics, so as to timely regulate the dosage of CLZ.