Methadose and downside

I have just recently started taking methadose, I was invovled in a motor vehicle accident in 1999. I experience severe pain everyday not always intolerable but sometimes excruciating pain and have suffered for years, until I went to a Pain Clinic that was a godscend to me. I can finally be halfway normal and do things I've only dreamed of and has helped me emoitionally as well, I am not so despressed. The only downside is that I become very constipated and have gained weight especially in the abdominal area. I'm not complaining because I am finally not suffering in pain or suicidal I feel great. But I am concerned that my abdominal area will continue to grow, I work out and do not over eat nor eat a lot of sweets any suggestions I'd appreciate it !