Common side effects of metformin hydrochloride include diarrhea, nausea, vomiting, bloating, fatigue, indigestion, abdominal discomfort, and headache. Less common side effects may include abnormal stools, hypoglycemia, muscle pain, dizziness, lightheadedness, nail abnormalities, rash, increased sweating, taste disturbances, chest discomfort, chills, flu-like symptoms, hot flashes, palpitations, and weight loss. Metformin can reduce the absorption of vitamin B12 in the intestines, but it rarely causes anemia. When taken within the therapeutic dosage range, metformin is rarely associated with lactic acidosis. Therefore, metformin remains a relatively safe first-line medication for managing blood sugar levels.