The study aimed to determine the prevalence of continuity of illicit drug use and its associated factors among Malay opioid users receiving methadone treatment in Kelantan, Malaysia. This cross-sectional study involved a total of 150 respondents. Sociodemographic characteristic of participants, level of religiosity, and quality of life were collected using the self-administered questionnaires. The prevalence of continuity illicit drug use was 38.7% (n = 58) by self-report and 40.7% (n = 61) by positive urine test result for drugs. Positive hepatitis C status and high quality of life in the psychological health domain were associated with low continuity of illicit drug use by self-reported data and urine drug test results. In conclusion, the continuity of illicit drug use among methadone program users is still high, especially among those on methadone therapy for less than 2 years.