For me it's metabolism problems. I was eating well under whats required for proper nutrition and still put on weight extremely fast. I don't know how you deal with it though. My way was to drop the med and use starvation. Obviously I'm not going to recommend that too anyone.

If you are eating healthy and proper portion sizes, getting in enough activity and still gaining I would seriously consider metabolism problems. I don't know how to correct that besides switching meds. There are foods you can eat and I'm sure other things to help speed up metabolism. I wish I knew them but I try not to engage in reading that kind of stuff for my own problems.