It won't have much much of an effect-other drugs such as amphetamines and coke increase your metabolism and reduce your appetite at the same time, leading to weight loss. Cannabis isn't really much of a stimulant, and the munchies are bound to put ant lost weight back on.

Cannabis is used medicanally to treat cancer patients who need to keep their food down and put their weight back on (after being treated with chemotherapy)-if it caused weight loss, treatment with cannabis wouldn't be a good idea.