Why is it important to withhold metformin after the parenteral administration of iodinated contrast?

Metformin, an oral hypoglycemic agent used in the treatment of type II non-insulin-dependent diabetes mellitus, is excreted solely through the kidneys. Patients who take metformin and have impaired renal function are at risk for developing potentially fatal lactic acidosis. Radiographic contrast may lead to acute renal failure and therefore puts patients taking metformin at risk for developing severe lactic acidosis. After iodinated contrast is given, metformin is held for 2 days, and then renal function is checked. If renal function is normal, metformin may be restarted.