They may cause weight gain as a side effect. Side effects are rare, and include hypoglycemia,stomach upset, loss of appetite, skin rash or itching, and liver function abnormalities. Special note should be made of chlorpropamide. It is extremely long acting and may not be suitable for some people. This wears off on its own but is frightening.
Metformin increases the sensitivity of liver and muscle to insulin.
It does not increase insulin secretion from the pancreas to any extent. It can be used in combination with a sulfonylurea drug if the SU drug alone is not sufficient. It has some unique advantages. It reduces triglycerides somewhat, and the average person loses about 5 pounds on the drug. It often works when other oral drugs fail.