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Pharmacologic class: Biguanide
Therapeutic class: Hypoglycemic
Pregnancy risk category B
FDA Box Warning
• Lactic acidosis is rare but serious (50% mortality) metabolic complication that can result from drug accumulation. Lactic acidosis is also linked to such conditions as diabetes mellitus and significant tissue hypoperfusion and hypoxemia. Lactic acidosis incidence in patients receiving drug is low; cases have occurred mainly in diabetics with significant renal insufficiency. Patients with unstable or acute heart failure at risk of hypoperfusion and hypoxemia are at increased lactic acidosis risk.
• Lactic acidosis risk rises with age and degree of renal dysfunction, and may decrease significantly through regular renal monitoring and by using lowest effective dosage. Perform careful renal monitoring, especially in elderly patients. Withhold drug promptly if patient develops condition linked to hypoxemia, dehydration, or sepsis. Avoid giving drug to patients with hepatic disease, as hepatic impairment may significantly limit lactate clearance. Caution patient against excessive alcohol use during therapy, as alcohol potentiates drug effects on lactate metabolism. Temporarily withdraw drug before intravascular radio-contrast study or surgical procedure.
• Lactic acidosis onset commonly is subtle and accompanied only by nonspecific symptoms. Instruct patient to notify physician immediately if symptoms occur; withdraw drug until situation is clarified.
• Suspect lactic acidosis in diabetic patient with metabolic acidosis who lacks evidence of ketoacidosis.
• Lactic acidosis requires emergency treatment. Discontinue metformin immediately and begin general supportive measures promptly. Prompt hemodialysis is recommended to correct acidosis and remove accumulated drug, and can lead to prompt symptom reversal and recovery.
Increases insulin sensitivity by decreasing glucose production and absorption in liver and intestines and enhancing glucose uptake and utilization
Oral solution: 100 mg/ml, 500 mg/5 ml
Tablets: 500 mg, 850 mg, 1,000 mg
Tablets (extended-release): 500 mg, 750 mg
Indications and dosages
➣ Adjunct to diet and exercise to improve glycemic control in type 2 (non-insulin-dependent) diabetes mellitus
Adults and children ages 17 and older: Initially, 500 mg P.O. b.i.d.; may increase by 500 mg/week, up to 2,000 mg/day. If patient needs more than 2,000 mg/day, give in three divided doses (not to exceed 2,500 mg/day). Alternatively, 850 mg P.O. daily, increased by 850 mg q 2 weeks, up to 2,550 mg/day in divided doses (850 mg t.i.d.). Extended-release tablets-500 mg/day P.O. with evening meal; may increase by 500 mg weekly, up to 2,000 mg/day. If 2,000 mg once daily is inadequate, 1,000 mg may be given b.i.d.
Children ages 10 to 16: 500 mg P.O. b.i.d. Increase in increments of 500 mg weekly to a maximum of 2,000 mg daily in divided doses.
➣ Concurrent use with sulfonylurea or insulin in type 2 diabetes mellitus
Adults and children ages 17 and older: If patient hasn't responded to maximum metformin dosage of 2,000 mg/day in 4 weeks, sulfonylurea may be added while metformin therapy continues at highest dosage (even if patient experienced primary or secondary failure on sulfonylurea). Adjust dosages of both drugs until glycemic control adequate. If response inadequate within 1 to 3 months of concurrent therapy, consider alternatives.
➣ Concurrent use with insulin in type 2 diabetes mellitus
Adults ages 17 and older: Continue current insulin dosage while starting metformin at 500 mg P.O. once daily. If response inadequate, increase metformin dosage by 500 mg after approximately 1 week and then by 500 mg weekly until glycemic control is achieved. Maximum metformin dosage is 2,500 mg. Optimally, decrease insulin dosage 10% to 25% when fasting plasma glucose level is below 120 mg/dl. Individualize dosage adjustments based on glycemic response.
• Elderly or debilitated patients
• Hypersensitivity to drug
• Acute or chronic metabolic acidosis (including diabetic ketoacidosis) with or without coma
• Underlying renal dysfunction
• Heart failure requiring drug therapy
Use cautiously in:
• renal impairment, myocardial infarction, cerebrovascular accident, hypoxia, sepsis, pituitary deficiency or hyperthyroidism, dehydration, hypoxemia, chronic alcohol use
• elderly or debilitated patients
• pregnant or breastfeeding patients
• children (safety not established).
• Administer with a meal.
• Make sure patient swallows extended-release tablets whole without crushing or chewing.
• Don't administer extended-release tablets to children.
• Know that drug is given with diet therapy, sulfonylureas, or both.
GI: diarrhea, nausea, vomiting, abdominal bloating
Metabolic: lactic acidosis
Other: unpleasant metallic taste, decreased vitamin B12 level
Drug-drug. Amiloride, calcium channel blockers, digoxin, morphine, procainamide, quinidine, ranitidine, triamterene, trimethoprim, vancomycin: altered response to metformin
Cimetidine, furosemide, nifedipine: increased metformin effects
Iodinated contrast media: increased risk of lactic acidosis
Drug-diagnostic tests. Urine ketones: false-positive results
Drug-herbs. Glucosamine: decreased glycemic control
Chromium, coenzyme Q10, fenugreek: additive hypoglycemic effects
Drug-behaviors. Alcohol use: increased metformin effects
• When switching from chlorpropamide, stay alert for hypoglycemia during first 2 weeks of metformin therapy; chlorpropamide may stay in body for prolonged time. Conversion from other standard oral hypoglycemics requires no transition period.
• Monitor blood glucose level closely. If it isn't controlled after 4 weeks at maximum dosage, oral sulfonylurea may be added.
• Monitor kidney and liver function tests, particularly in elderly patients.
• Assess hematologic parameters and vitamin B12 levels at start of therapy and periodically thereafter.
☞ Watch for signs and symptoms of lactic acidosis. Stop drug if acidosis occurs. To aid differential diagnosis, check electrolyte, ketone, glucose, blood pH, lactate, and metformin blood levels.
• Periodically monitor glucose and glycosylated hemoglobin levels to evaluate drug efficacy.
• Teach patient about diabetes and importance of proper diet, exercise, weight control, and blood glucose monitoring.
• Inform patient that drug may cause diarrhea, nausea, and upset stomach. Advise him to take it with meals to reduce these effects, and tell him that adverse effects often subside over time.
☞ Teach patient to recognize and immediately report signs and symptoms of acidosis, such as weakness, fatigue, muscle pain, dyspnea, abdominal pain, dizziness, light-headedness, and slow or irregular heartbeat.
• Advise patient to report changes in health status (such as infection, persistent vomiting and diarrhea, or need for surgery). These may warrant dosage decrease or drug withdrawal.
• As appropriate, review all other significant and life-threatening adverse reactions and interactions, especially those related to the drugs, tests, herbs, and behaviors mentioned above.