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Long‐term sample stability of five atypical antipsychoticdrugs risperidone, paliperidone, clozapine, quetiapine and olanzapine and the antidepressant drug mirtazapine in serum was studied by use of a newly developed and validated analytical method based on solid‐phase extraction and liquid chromatography–tandem mass spectrometry. Ascorbic acid was used as an antioxidative agent to stabilize olanzapine during storage and sample preparation. We assessed analyte stability on long‐term storage in serum samples at 25°C, 5°C, −20°C and −80°C, and during five freeze–thaw cycles. Analytes were stable for 23 days at room temperature except for olanzapine and mirtazapine (17 days). All analytes were stable for at least 30 days at 5°C. All analytes were stable for 270 days at −20°C, except for paliperidone and mirtazapine with 60 days and 180 days, respectively. All analytes were stable for 270 days at −80°C. Furthermore, all analytes were stable for five freeze–thaw cycles. We recommend storage at −80°C when samples drawn for analysis of antipsychotic drugs are stored for more than 60 days, whereas a temperature of −20°C is sufficient for storage less than 60 days. [ABSTRACT FROM AUTHOR]