Methadone maintenance treatment might be commonly associated with lying for several causes or manipulating psychiatrists and therapists by a number of patients. Deceptive patients tend to manipulate their psychiatrists for multiple causes. This study aims to improve clinicians’ therapeutic decision-making by identifying the reasons for increasing daily methadone maintenance dosage among deceptive patients. One hundred ninety-six patients undergoing the Methadone Maintenance Treatment (MMT) with no statistically significant difference between the overall Addiction Severity Index (ASI) scores at different doses of methadone (< 60 mg/d and ≥ 60 mg/d) and Ahvaz Reality Distortion Inventory (ARDI) scores > 30, were examined in the current qualitative study with multiple semi-structural interviews about the reasons for increasing daily methadone maintenance dosage. The investigation results revealed that the most common reasons for increasing daily methadone maintenance dosage among deceptive patients were opium craving, patient willingness to feel euphoria, fear of the withdrawal signs, earn money through the sale of surplus methadone, improve the symptoms of physical and psychiatric comorbidity, forgetting painful memories, curiosity, the influence of others, sexual issues, feeling of well-being, and appearance changes. Given these reasons, any increase in daily methadone maintenance dosage is not necessarily accompanied by improvement in the clinical condition of patients. However, clinicians can make the most appropriate therapeutic decision by putting the psychological assessments and clinical interviews into play.