This is such an interesting question as people do not typically think of marijuana promoting weight loss. The short answer is that cannabis can help with weight loss in certain circumstances.
As most people know, cannabis is most often used for the opposite effect, to promote the 'munchies'. Cachexia, or what is known as wasting disease, can be a side effect of medical conditions like cancer or AIDS. Cannabis came into more popular cultural/medical acceptance when cancer and AIDS patients started using marijuana to help them eat as well as deal with pain and other side effects from their illnesses.
A recent study by the American Journal of Medicine found that marijuana consumers were 'found significant associations between marijuana use and smaller waist circumferences.' Their findings also indicated that consistent consumers (3-5x per week) had 16 percent lower fasting insulin levels. It seems that marijuana promotes glucose absorption as well as the 'good cholesterol'.
Not all strains will promote weight loss however and it is important to remember a healthy lifestyle is just as important. Some strains do seem to suppress appetite and the strains that do so are the ones that contain THCV, or tetrahydrocannabivarin. This is a lesser known compound which actually inhibits the cannabinoid THC. Be wary, you will still get high with THCV though! Many of the strains that have THCV originate from Africa. It also tends to be within sativa dominated strains.
Some of the most popular strains that contains THCV are Durban Poison, Willie Nelson and Skunk#1. Be sure to ask your local bud tender when you purchase your flower for the strain's profile. Lab results should reveal this as well. Although it is easier to find flowers that contain THCV you can also make oils, edibles and extracts from these strains.
I hope this helps!