The most common and less serious side effects of metformin are bloating, gas, stomach pain, weakness, diarrhea, constipation, heartburn, sneezing, headaches, cough, flushing of the skin, vomiting, runny nose, nausea and muscle pain. If you have chest pain or a rash, seek immediate medical help. Also call for emergency help if you have any symptoms of lactic acidosis: weakness, cold feeling, increasing sleepiness, slow heart rate, shortness of breath, muscle pain, stomach pain, light-headedness, and fainting. Hives, swelling of your face, lips, tongue or throat, and difficulty breathing could be a sign of an allergic reaction. Call your doctor if you begin gaining weight, notice swelling or develop flu-like symptoms. Metformin may cause hypoglycemia, which is low blood sugar. Symptoms for hypoglycemia are dizziness, shakiness, irritability, nervousness, changes in mood or behavior, headaches, weakness, numbness or tingling around your mouth, hunger, pale skin, and clumsy movements. If you do not get treated for hypoglycemia, it can become more severe and graduate into confusion, followed by seizures and loss of consciousness.
You can also develop hyperglycemia, which is high blood sugar. Symptoms are frequent urination, extreme hunger, thirst, blurred vision and weakness. If not treated, it can develop into a more serious life-threatening diabetic ketoacidosis. Symptoms are nausea, vomiting, dry mouth, fruity smelling breath and lapsing in unconsciousness. Because metformin is used in combination drugs, some of those other active ingredients may also have side effects.