I would recommend seeing a dietician specializing in diabeties and/or obesity to help you lose weight in a healthy manner. I would also recommend getting a second opinion. Metformin is mainly used for managing Type 2 diabetes and the risk for cardiovascular complications in overweight people, without causing large changes in weight gain or weight loss. Metformin can cause diarrhea and vomiting, as you mentioned. You can either give it another shot since metformin is an extremely effective medication or you can tell the doctor of your past experience with it and perhaps have them prescribe an alternate medication.