metformin (Rx)

Black Box Warnings

Lactic acidosis is a rare, but potentially severe, consequence of therapy with metformin; it is characterized by elevated blood lactate levels (>5 mmol/L), decreased blood pH, electrolyte disturbances with an increased anion gap, and an increased lactate/pyruvate ratio; when metformin is implicated as the cause of lactic acidosis, metformin plasma concentrations >5 mcg/mL are generally found

Risk factors for metformin-associated lactic acidosis include renal impairment, concomitant use of certain drugs (eg, carbonic anhydrase inhibitors such as topiramate), age 65 years old or greater, having a radiological study with contrast, surgery and other procedures, hypoxic states (e.g., acute congestive heart failure), excessive alcohol intake, and hepatic impairment; if metformin-associated lactic acidosis is suspected, immediately discontinue

Patients with CHF requiring pharmacologic management, in particular those with unstable or acute CHF who are at risk for hypoperfusion and hypoxemia, are at an increased risk for lactic acidosis; the risk for lactic acidosis increases with the degree of renal dysfunction and the patient’s age

Do not start in patients aged 80 years or older unless CrCl demonstrates that renal function is not reduced, because these patients are more susceptible to developing lactic acidosis; metformin should be promptly withheld in the presence of any condition associated with hypoxemia, dehydration, or sepsis

Should generally be avoided in patients with clinical or laboratory evidence of hepatic disease; patients should be cautioned against excessive alcohol intake, either acute or chronic, during metformin therapy because alcohol potentiates the effects of metformin on lactate metabolism

Discontinue metformin at the time of or before an iodinated contrast imaging procedure in patients with an eGFR between 30-60 mL/minute/1.73 m²; in patients with a history of liver disease, alcoholism, or heart failure; or in patients who will be administered intra-arterial iodinate contrast

The onset of lactic acidosis often is subtle and accompanied by nonspecific symptoms (eg, malaise, myalgias, respiratory distress, increasing somnolence, nonspecific abdominal distress); with marked acidosis, hypothermia, hypotension, and resistant bradyarrhythmias may occur; patients should be instructed regarding recognition of these symptoms and told to notify their physician immediately if the symptoms occur; metformin should be withdrawn until the situation is clarified; serum electrolytes, ketones, blood glucose, and, if indicated, blood pH, lactate levels, and even blood metformin levels may be useful

Once a patient is stabilized on any dose level of metformin, GI symptoms, which are common during initiation of therapy, are unlikely to be drug related; later occurrences of GI symptoms could be due to lactic acidosis or other serious disease

Lactic acidosis should be suspected in any diabetic patient with metabolic acidosis who is lacking evidence of ketoacidosis (ketonuria and ketonemia); lactic acidosis is a medical emergency that must be treated in a hospital setting; in a patient with lactic acidosis who is taking metformin, the drug should be discontinued immediately and general supportive care measures promptly instituted; metformin is highly dialyzable (clearance up to 170 mL/min under good hemodynamic conditions); prompt hemodialysis is recommended to correct the acidosis and to remove the accumulated metformin; such management often results in prompt reversal of symptoms and recovery