I Have Been Taking Duromine For About 1 Year, And

I have been taking duromine for about 1 year, and it has kept my weight steady will no ill effects. I recently had my gallbladder removed, and although I am taking duromine again, I am putting on weight, and my appetite has increased dramatically. I have put on at least 10 kgs.
Why isn't duromine working for me.