Assessment of a methadone substitution and maintenance treatment program in Colombia using the OTI (Opiate Treatment Index) instrument. Descriptive study with a 50-patient sample attended in the period 2007- 2008 for their heroin addiction. The OTI instrument was implemented to assess drug use, HIV risk-taking behavior, social functioning, criminality, health status and psychological functioning. Men accounted for 74% of the sample, women for 26%. Mean age was 22, with 64% in the 20-24 age group. Mean reported age at onset of heroin use was 18, with a range of 13-28. In 38% of the sample (19 patients) comorbidity with other mental disorders was found, 47% of these being personality disorders. Median reported current methadone dose was 40 mg/day, with a range of 15-80 mg/day. According to the OTI subscales, HIV risk-taking behavior was low-moderate, while social functioning, criminality and health status were low-risk. In the case of psychological adjustment, a low-moderate risk score was obtained for the 0-12 month treatment group, and a moderate risk score for the 12-24 and ≥ 24 month groups. Starting age for heroin use in this population is early . In general, the risk assessed in the OTI areas is low, possibly due to problematic heroin use being in its early stages in Colombia, but comorbidity with personality disorders is high.