Methadone Maintenance Therapy (MMT) has been accepted as a gold-standard treatment for opioid therapy. It may associate with adverse effects. This study aimed to affect methadone on serum lipid profile, leptin levels, and liver enzymes in male and female rats. 41 Wistar rats weighing 200-300gr were randomly assigned into four groups, including two methadone treatments and two control groups, both male and female, that received 5mg/kg methadone or 1cc normal saline daily for 8 weeks respectively by gavage method. All animals were weighed weekly. Fasting blood sugar (FBS) was measured by a glucometer, and blood samples were taken by cardiac puncture for triglyceride, low-density lipoprotein (LDL),high-density lipoprotein (HDL), total cholesterol, Aspartate transaminase (AST), Alanine transaminase (ALT), alkaline phosphatase (ALP) and leptin levels measurement after 12h fasting. One-way ANOVA showed no significant difference in mean FBS, total cholesterol, triglyceride, and LDL levels among the four groups. Moreover, there was a statistically significant difference in the mean of HDL, ALT, AST, and ALP levels. Furthermore, repeated measures ANOVA indicated a significant increase in body weight of rats during 8 weeks. Our findings indicated changes in some metabolic effects associated with methadone treatment in a gender-dependent manner.