I have a theory. Every action has an equal and opposite reaction (Einstein) so this applies to weight gain as well. For example if an experiment was done in say a very poor country such as Ethiopia by taking a group of people who have limited access to food and were showing malnutrition.

Give this group a course of weight gaining drugs and study the results without increasing food intake . Would they gain weight? They may gain water through water retention but would they pack on 20lb? I cant see how that would be possible

Limit our food intake to match the energy we burn. When i was at my worst I was couch bound and eating 3 square meals a day the result was always going to be unpleasant. PN ers have it tough that way. These days I eat 1 small meal a day and snack on fruit and am renaming neutral

Digressing... The answer to the question of which drugs I would say any opiate. As I have mentioned before, ever seen a fat drug addict