If you were to attempt to use drugs/medication to increase either appetite or weight gain, what would be the most practical one. This is not intended for IRL use, but for a story I am in the process of writing. I like my stories to be as realistic as possible. However, if there is a chance for extremely bad side affects(that aren't weight gain related) I can ignore them.

You could literally wiki "Appitite Stimulant" and read through the list it provides, from obvious things like weed to specific neurotransmitter stimulants.