Opioid use disorder is a pattern of problematic opioid use, leading to impaired functioning or clinically significant suffering. The Treatment options include symptomatic withdrawal, withdrawal with selective alpha-2 noradrenergic agonist mimetics (clonidine or Lofexidine), and finally, withdrawal with opioid agonists (methadone and Buprenorphine), which remains the most effective in reducing morbidity and mortality. The main objective of our work was to study the long-term effect of methadone substitution therapy on the quality of life of patients undergoing outpatient treatment in Morocco, then to determine the factors influencing this quality of life, before concluding with recommendations for improving the overall management of the patient. The total number of patients responding to the questionnaire was 60 participants. Social and environmental components were moderately improved in methadone-treated patients. Negative feelings such as melancholy, despair, anxiety or depression were present in the majority of our participants, with varying frequencies, with a significant alteration in the psychic component. For several years, quality of life has been a major preoccupation of healthcare professionals in a bio- psycho-social approach. In this vision of care, quality of life should now be part of the clinical criteria for monitoring patients on methadone.