Metformin is inappropriate for patients with diabetic coma and ketoacidosis, or for those with severe infection, trauma, or other severe conditions where Metformin is unlikely to control the hyperglycemia; insulin should be used in such situations. Metformin should not be given to patients with even mild renal impairment, as it may predispose patients to lactic acidosis and renal function should be monitored throughout therapy. Dehydration may contribute to renal impairment. Conditions associated with hypoxia, such as acute heart failure, recent myocardial infarction, or shock, may increase the risk of lactic acidosis. Other conditions that may also predispose to lactic acidosis in a patient taking a Metformin include excessive alcohol intake and hepatic impairment. Biguanides should be temporarily stopped 48 hours before examinations using contrast media because of the risk of contrast media-induced renal impairment; biguanides should be withheld for at least 48 hours after the examination, and until normal renal function is confirmed. Insulin is preferred for the treatment of diabetes in pregnancy. Owing to the possibility of decreased vitamin B2 absorption, annual monitoring of vitamin B2 concentrations is advisable during long-term treatment.