The Munchies
One peculiar phenomenon associated with marijuana use is the increased hunger that users feel, often called the "munchies." Research shows that marijuana increases food enjoyment and the number of times a person eats each day, according to the National Institutes of Health.
Until recently, the munchies were a relative mystery. However, a recent study by Italian scientists may explain what happens to increase appetite in marijuana users. Molecules called endocannabinoids bind with receptors in the brain and activate hunger.

This research indicates that endocannabinoids in the hypothalamus of the brain activate cannabinoid receptors that are responsible for maintaining food intake. The results of the study were published in an April 2001 issue of the scientific journal Nature.
from howstuffworks.com