There skinny genes, fat genes, good genes.. ect.
Usually a guy that has fat genes has no issue putting on size, whether that is muscle or fat. Usually a guy that has skinny genes has a hard time putting on any size at all.. and won't get fat no matter how much or what he eats.
So if you are saying you can't put on muscle, and just get fat.. which might be some shitty genetics you have.. or you just are not training and eating right for your body type. What does your daily diet look like? and what style of lifting do you do?
I'd suggest getting those squared away before you start throwing any kind of drug at it. The drugs will help obviously, but once you come off them.. you will just fall back to where you began if your diet isn't fixed.