You only gain weight on methadone if you eat like crap, and you don't exercise. Even though I was an opiate user for over six years, I also became a fitness fanatic. Kind of ironic, I know.
I have decided to quit using opiates for good since it recently got out of hand, and decided to get on methadone. I have noticed a lot of water retention from the drug, but no fat at all, I have actually lost fat because of my training while on methadone.
Don't sit here and blame a drug because of your weight gain methadone just simply doesn't allow you to get high so you are not in a calorie deficit because you are not eating. You just have a poor diet, and don't exercise is enough.
Most people jump and blame their meds, when most of the time it's their crappy diet. I have been heavy my whole life even while a drug addict. It wasn't until I started getting into fitness that I am now a normal weight. Methadone has nothing directly to do with fat gain. It isn't like SSRI's that actually lower your metabolism. and pack on pounds.