“Acute marijuana use is classically associated with snacking behavior (colloquially referred to as “the munchies”). In support of these acute appetite-enhancing effects, several authorities report that marijuana may increase body mass index in patients suffering from human immunodeficiency virus and cancer…
Marijuana is a clinically controversial substance, but one potential medical benefit may be weight gain. According to available studies, appetite stimulation as well as weight gain may occur in patients with physical debilitation due to HIV/AIDS and/or cancer.
As for the effects of marijuana on body weight in the general population, use appears to be associated with a lower body mass index.
…marijuana may genuinely be a regulatory compound, increasing weight in those with low weight, but not in those who are normal or overweight.”