"Munchies" or "The Munchies" are a common and well-known effect of marijuana usage, usually treated with a great deal of comic levity, both by marijuana consumers and their detractors. Aside from stimulating the consumption of low-nutrition snack food, stimulation of the appetite is probably the foremost medical use of marijuana.
That being said, "The Munchies" are a complicated medical and social phenomena. Although even my staid Merck Index from 1989, before medical marijuana became an accepted practice, lists "Anti-Emetic" as the therapeutic use for Tetrahydrocannabinoids, there is a big leap from a drug biologically blocking emesis to a drug causing people to wish they had a nutter butter dipped in mint fudge.
Part of the confusion comes from the fact that while THC, in its many varieties, is the key part of marijuana's psychoactive effects, there are many different chemicals present in marijuana, and these vary from strain to strain, and especially within the two major species of the Cannabis genus, sativa and indica. The way these chemicals are metabolized by the body are also different from user to user, so there is a gigantic difference in marijuana's objective and subjective effects, including with things like "the Munchies".
As far as I know (although I am not an expert), the physiological mechanism of marijuana's appetite stimulating effects are not understood as a straight-forward cause and effect type thing. For example, amphetamine and related drugs suppress appetite by activating the fight or flight response, which turns off non-essential systems such as digestion. Marijuana's appetite stimulating effects are not as simple as this.
On top of this, and even more important, is the fact that marijuana is a drug whose use is very informed by the individual and group psychology of those who consume it. One of the best descriptions for this that I have read is that marijuana is an "active placebo". What this means for "the munchies" is that while marijuana might have a purely physiological reason for increasing appetite, it also derives much of its power from the fact that users think it will do so. And users will do so even more when they are surrounded by other like-minded people. This is especially notable when different sub-cultures' descriptions of marijuana are compared. For example, I have heard much hip-hop music extolling the pleasures of cannabis consumption, but getting the munchies is rarely (if ever) mentioned as part of the experience. When Method Man is smoking blunts, does he not get hungry, or does he just not accentuate that fact when he reports it in his music?
Another issue is where the "hunger" of "the munchies" comes from. Is it the hunger of the stomach, that is, the desire to have sustenance, or is the hunger of the mouth, an interest in textures and tastes? Or is it a combination of both?
So, in conclusion, or perhaps in lack of conclusion, although the connection between marijuana consumption and the desire to eat is something that there is rich anecdotal evidence for, and something that medical science has endorsed, the exact connection between the chemical and biological mechanism of this action, and the psychological and sociological context in which it occurs, is something that is unclear.
Why don't they make mint fudge covered nutter butters?