Abstract: Background and Objectives: Cholecalciferol plays an important role in the development of brain function and neuropsychiatric disorders, especially psychological disorders. The objective of this study was to observe the differencesin25(OH)D serum levels and Brief Psychiatric Rating Scale (BPRS) scores in individuals who use amphetamine-type stimulants (ATSs) after receiving supplementation with cholecalciferol 1000 IU per day. Materials and Methods: This study had a pre and post-test design with consecutive sampling. Cholecalciferol 1000 IU per day was given to 25 ATS users (intervention/I group), while the other 25 individuals, who composed the control group (C group), did not receive any cholecalciferol supplementation for 42 days. On laboratory tests, the serum level of 25(OH)D is categorized as normal if it is within 54-90 ng mL–1. Psychosis was evaluated with the BPRS questionnaire. Results: There was a significant difference in the serum level of 25(OH)D after the intervention between the groups (I group: 23.37±4.20; C group: 20.33±4.04; p = 0.012) and the increase in the serum level of 25(OH)D was greater in the I group than that of the C group. The result also showed a significant difference in BPRS score after the intervention between the groups [I group: 25(24-27); C group: 27(26-29); p<0.001]. The decrease in the BPRS score in the I group was larger than that in the C group. Conclusion: The administration of cholecalciferol 1000 IU per day increased the 25(OH)D serum level and cholecalciferol supplementation in subjects using ATSs led to a significant reduction in the BPRS score.