Background: Methadone is a synthetic opioid and receptor agonist morphine; thus, its consumption has the effects and side effects of opioid. Methadone maintenance treatment (MMT) is used as an alternative treatment for people who are suffering from substance abuse and do not have the ability to withdraw. Despite its benefits, this drug also has side effects. The purpose of this study was to investigate the effects of methadone treatment on sexual function, sleep, and weight after 6 months. Methods: The study subjects consisted of 200 patients who had referred to the Methadone Clinic of Shahid Beheshti Training Hospital, Kerman, Iran, during a 6-month period and were treated using MMT. Data collection tools consisted of the demographic questionnaire, Pittsburgh Sleep Quality Index (PSQI), and Arizona Sexual Experience Scale (ASEX). The questionnaires were completed by the participants before and 6 months after the treatment. Findings: The results of this study showed that methadone consumption has significant effects on sexual dysfunction, sleep disorders, and weight gain. Conclusion: The consideration of the side effects of MMT can result in consumers’ commitment to the treatment.