I was the same way...when my addiction really got serious I lost a ton of weight because I never wanted to eat and I couldn't sit still for 5 minutes. Now I'm off of them I have put on 25 pounds that I'm having a he!! Of a time loosing. I don't think it's the medication that's causing the weight gain. The meds can cause the constipation though.
Drugs affect us in so many different ways. I became a string bean on them. I have gained weight off of them. But some people loose weight on them. Maybe your one of the ones who will loose weight. Just keep up the fight. Supporting you.