Can smoking marijuana cause weight loss?

Uncertain: Two research studies that made the news in 2011 found 14%-17% of marijuana users (3 or more times/week) were obese. Presumably, many other mj users were overweight but not obese. Their findings in people who did not use mj showed 22%-25% of non-users were obese. Perhaps people who tend to become mj users are less obese to start with, or maybe mj users use mj instead of food to soothe their stress.

Usually the opposite: Marijuana has a well-known association with an increased desire to eat (commonly called getting the munchies), and often an increased desire to eat sweets. Regular use of marijuana or tetrahydrocannabinol (thc, the active ingredient in marijuana) usually increases appetite and increases weight gain. That's one of the reasons it is advocated for cancer patients and anorexics.

Rarely in some: Smoking marijuana causes a variety of changes neurobiologically as well as behaviorally and while many would be expected to gain weight there would be a subset of people that might experience weight loss. Different mechanisms may be involved although i could see psychomotor retardation, new onset apathy, paranoia, psychosis and increased anxiety as being possible routes to weight loss. Stay safe.