Change in plasma concentration of clozapine and norclozapine following a switch of oral formulation

Background: Clozapine formulation has been shown to affect plasma concentrations of clozapine and norclozapine. Changes in formulation might result in toxicity or treatment failure.Methods: We identified, from electronic records, patients who switched from clozapine tablets to oral liquid or vice versa and compared plasma concentrations before and after the switch.Results: We identified 13 patients with 85 blood samples who changed formulation of clozapine. Overall mean standardized clozapine plasma level was 0.67 mg/l daily dose on liquid and 0.87 mg/l daily dose on tablets ( p = 0.035).Conclusion: Use of clozapine liquid results in lower plasma clozapine concentration than the same dose of tablets.