There is weight gain associated with
Methadone, mainly because the drug causes you to crave sweets. Narcotics usually causes one to lose weight because it effects your appetite and makes you lethargic, so
Percocet itself shouldn't be causing weight gain. Like anything, limit your caloric intake and exercise, and if you're on methadone, keep your mouth well hydrated, or you could lose your teeth, it inhibits saliva flow which washes your teeth, water alone isn't enough, try and force yourself to salivate.