Methadone Hydrochloride and severe constipation

I have been on methadone for about 4 years. I have gained about 60 pounds within 2 yrs of being on the program. I never had a weight problem in my whole life until now. Dieting and exersize did nothing.Also have severe constipation.Ive tried about 15 diffirent drugs to help the constipation. Most of them helped the issue but still leaving me with 1 or 2 bowl movements within 8 days.If I dident take anything for it Id wind up in the er with no bowl moement at all in severe pain.Also my stomache is huge, at times I look about 5 months pregnant. If anyone has any advice for the problems I am having please let me know.