The side effects when taking metformin are heartburn, weight loss, constipation, gas, diarrhea, bloating, stomach pain, vomiting, nausea, weakness, asthenia, muscle pain, upper respiratory tract infection, low blood levels of vitamin B-12, abdominal pain and (GI complaints), lactic acidosis (rare).
For most people who take metformin to control blood sugar they may not experience side effects but for some people who take metformin they may have side effects.
If you do experience side effects when taking metformin it would be a good idea to let your doctor know about the side effects.
However the side effects when taking metformin are usually nothing serious your doctor may be able to take you off of metformin and put you on something else to control your blood sugar.