Cyproheptadine has been used as an appetite stimulant to increase body weight in various population. This study aimed to determine the effect of cyproheptadine on weight changes in Thai patients. A retrospective longitudinal study was conducted in adult patients who were prescribed with cyproheptadine, having body weight records at 2 times consecutively during 12-month period at the medical outpatient department, the Police General Hospital, Thailand. Of 125 participants, 69.6% were females and the mean age was 78.38 (SD ± 11.68) years. Hypertension, dyslipidemia and diabetes mellitus were the most common underlying conditions. The mean body mass index (BMI) at 1st visit was 21.16 (SD ± 3.64) kg/m2. The mean body weight at 1st and 2nd visit were 52.46 (SD ± 11.11) kg, and 52.61 (SD ± 10.98) kg, respectively. Overall, there was no significant change in body weight between two visits. In underweight patients (BMI < 18.5 kg/m2), the mean BMI decreased significantly in the 2nd visit compared to 1st visit (p = 0.044). At the 2nd visit, older age and high-density lipoprotein cholesterol levels were negatively associated with body weight (p < 0.05). The polypharmacy (odds ratio (OR), 0.778; 95% confidence interval (CI), 0.616 – 0.982), the presence of hypertension (OR, 0.022; 95%. CI, 0.001 – 0.390) and low-density lipoprotein cholesterol levels (OR, 0.969; 95% CI, 0.942 – 0.996) were also negatively associated with abnormal BMI. Cyproheptadine might not improve the body weight of patients in this population. The factors associated with lower body weight in this study may be helpful in further research.