Common metformin side effects include diarrhea, nausea, and vomiting. It can also cause fatigue, weight loss, and low vitamin B12 levels. While uncommon, metformin can also cause blood glucose (sugar) to drop too low and lactic acidosis. In severe cases, these side effects are medical emergencies. Metformin should be taken with meals to help reduce stomach or bowel side effects that may occur during the first few weeks of treatment. Swallow the tablet or extended-release tablet whole with a full glass of water. Metformin can cause side effects in some people. Learn about its mild and serious side effects, as well as factors that can increase your risk. Metformin commonly causes side effects like nausea, diarrhea and bloating. More serious side effects involve low blood pressure and lactic acidosis. Many of the warning signs are similar to some metformin side effects, such as stomach pain, dizziness, and weakness. Others are numbness or a cold feeling in your limbs, or changes in your. Learn about the side effects of metformin, from common to rare, for consumers and healthcare professionals. Common side effects of metformin hydrochloride tablets include diarrhea, nausea, vomiting, wind (flatulence), indigestion, and headache. These side effects occurred in 5% or more of patients in the US clinical trial of using metformin for type 2 diabetic patients.