This is something I've been wondering for a long time. Not that it's my problem, I weigh some 110kilos (240 lb) 
Anyway, let's do a quick check:
amphetamines / pretty much any CNS speed - hugely decrease appetite.
opiates - decrease
lsd, psylocibin / other psychedelics - decrease
all the RCs Ive heard about - decrease
steroids - decrease
benzos - I think those are neutral
cannabis - huge increase.
I can't honestly think of any other drug but this can't be true, I don't know all drugs.
Also have you ever met a fat addict? Excluding addiction to food itself lol.

Anyway, let's do a quick check:
amphetamines / pretty much any CNS speed - hugely decrease appetite.
opiates - decrease
lsd, psylocibin / other psychedelics - decrease
all the RCs Ive heard about - decrease
steroids - decrease
benzos - I think those are neutral
cannabis - huge increase.
I can't honestly think of any other drug but this can't be true, I don't know all drugs.
Also have you ever met a fat addict? Excluding addiction to food itself lol.