| Clozapine-induced weight gain is a common adverse drug reaction that leads to obesity, metabolic syndrome and cardiovascular disease. Objective= To study the body weight changes in Thai psychiatric patients who received clozapine for 24 months. Methods= This retrospective observational study was conducted by reviewing the medical records from Prasrimahabhodi psychiatric hospital. The inclusion criteria were schizophrenia patients, diagnosed with F20-F29 =ICD-10, aged 18-59 years, and continuously received clozapine for two years. Descriptive statistics were used to analyze the characteristics of subjects. ANOVA was used to compare the average weight changes at month 0, 3, 6, 9, 18 and 24 after start using clozapine. Results= Of 191 subjects with mean age 38.6 ± 9.6years, 129 (67.5%) were male. The prevalence of weight gain more than 7% of the original body weight observed at month 3, 6, 9, 12, 18 and24 were27.23, 16.75, 29.32, 32.46, 30.37 and 25.13; respectively. Moreover,the average weight change (SD) at month 3, 6, 9, 12, 18 and24 were+4.2 (6.5) ,+2.5(4.3),-8.9 (22.5), -7.3 (23.3), -7.8 (23.5)and-7.8 (20.1) kg; respectively. Statistically significant difference of weight change over time was not observed. Conclusions= The highest prevalence of subject gained weight more than 7% of the original body weightat month12. The highest mean weight change occurred within 3 months after starting clozapine, and gradually decreased over time. Our results were contradicted to the previous studies in western countries due to some factors such as race, food. Prospective study should be conducted to investigate causes of weight loss after six months after clozapine initiation |