You shouldn't worry unnecessarily. I was on steroids and had no increase in appetite and actually lost weight. I did get other side effects, but after a while my doctor was able to prescribe things to counteract the worst of them. There's not really a good way to predict which side effects you get or how badly. If you do gain weight on steroids, it will either be water weight (the body retaining extra fluid), which is minimal and will quickly go after the steroids are stopped, or it is due to an increased appetite. If you are able to resist your food cravings, or are able to satisfy them with low calorie foods, you won't gain weight. I know that's easier said than done, but it's definitely possible to take the medications without weight gain.