Full-text article. Also available at https://www.sciencedirect.com/science/article/pii/S2665910719300374Trends of continual drug use while under treatment have been reported worldwide. This paper assesses opioid use among patients receiving treatment in a methadone clinic for opioid addiction. A total of 126 urine samples were analyzed using LC–MS/FTMS. About 16% of patients were found to have a high prevalence level of opioids use. Prevalence level was significantly associated with the duration of treatment (p = 0.0121) and age (p = 0.0328). About 125/126 patients were found to have remnants of opioids of abuse. Therefore, measures should be taken to overcome continual use of opioids among clients.