METFORMIN

(met-for'min)
Fortamet, Glucophage, Glucophage XR, Glumetza, Riomet
Classifications: hormones & synthetic substitutes; antidiabetic agent; biguanides
Pregnancy Category: B

Availability

500 mg, 850 mg, 1000 mg tablets; 500 mg, 750 mg, 1000 mg sustained-release tablets; 100 mg/mL oral solution

Actions

Biguanide oral hypoglycemic agent. Unlike sulfonylureas, biguanides do not stimulate the release of insulin from the beta cells of the pancreas. Mechanism of action is thought to be due to both increasing the binding of insulin to its receptor and potentiating insulin action.

Therapeutic Effects

Improves tissue sensitivity to insulin, increases glucose transport into skeletal muscles and fat, and suppresses gluconeogenesis and hepatic production of glucose, thus lowering blood glucose levels.

Uses

Treatment of type 2 diabetes mellitus in patients not controlled with diet alone. May be used with an oral sulfonylurea.

Contraindications

Hypersensitivity to metformin; hepatic or cardiopulmonary insufficiency; alcoholism; concurrent infection; acute MI, cardiogenic shock; diabetic ketoacidosis; hypoxemia, lactic acidosis; radiographic contrast administration; renal disease, renal failure, renal impairment; sepsis; surgery; children <10 y, lactation.

Cautious Use

Previous hypersensitivity to phenformin or buformin; anemia; coma; dehydration, diarrhea; older adults; ethanol intoxication; fever; gastroparesis, GI obstruction; heart failure; hyperthyroidism, pituitary insufficiency; polycystic ovary syndrome; trauma, emesis; pregnancy (category B).

Route & Dosage

Type 2 Diabetes Mellitus
Adult: PO Start with 500 mg q.d. to t.i.d. or 850 mg q.d. to b.i.d. with meals, may increase by 500–850 mg/d every 1–3 wk (max: 2550 mg/d); or start with 500 mg sustained-release with p.m. meal, may increase by 500 mg/d at p.m. meal qwk (max: 2000 mg/d)

Administration

Oral

Ensure that extend-release tablets are not crushed or chewed. They must be swallowed whole.
Use a calibrated oral syringe or container to measure the oral solution for accurate dosing.
Give with or shortly after main meals.
Withhold metformin 48 h before and 48 h after receiving IV contrast dye.
Make dose increment, if needed, at 2- to 3-wk intervals.
Store at 15°–30° C (59°–86° F).

Adverse Effects (1%)

CNS: Headache, dizziness, agitation, fatigue. Metabolic: Lactic acidosis. GI: Nausea, vomiting, abdominal pain, bitter or metallic taste, diarrhea, bloatedness, anorexia; malabsorption of amino acids, vitamin B₁₂, and folic acid possible.

Interactions

Drug: Captopril, furosemide, nifedipine may increase risk of hypoglycemia. Cimetidine reduces clearance of metformin. Concomitant therapy with azole antifungal agents (fluconazole, ketoconazole, itraconazole) and oral hypoglycemic drugs has been reported in severe hypoglycemia. iodinated radiocontrast dyes can cause lactic acidosis and acute kidney failure. Amiloride, cimetidine digoxin, dofetilide, midodrine, morphine, procainamide, quinidine, quinine, ranitidine, triamterene, trimethoprim, or vancomycin may decrease metformin elimination by competing for common renal tubular transport systems. Acarbose may decrease metformin levels. Iodinated contrast dyes may cause lactic acidosis or acute kidney failure. Herbal: Garlic, ginseng may increase hypoglycemic effects.

Pharmacokinetics

Absorption: 50–60% of dose reaches systemic circulation. Peak: 1–3 h. Distribution: Not bound to plasma proteins. Metabolism: Not metabolized. Elimination: Excreted in urine. Half-Life: 6.2–17.6 h.

Nursing Implications

Assessment & Drug Effects

Lab tests: Obtain baseline and periodic kidney and liver function tests; drug contraindicated in the presence of renal or hepatic insufficiency. Monitor blood glucose and HbA_1C, and lipid profile periodically.
Monitor known or suspected alcoholics carefully for decreased liver function.
Monitor cardiopulmonary status throughout course of therapy; cardiopulmonary insufficiency may predispose to lactic acidosis.

Patient & Family Education

Be aware that hypoglycemia is not a risk when drug is taken in recommended therapeutic doses unless combined with other drugs which lower blood glucose.
Report to physician immediately S&S of infection, which increase the risk of lactic acidosis (e.g., abdominal pains, nausea, and vomiting, anorexia).
Do not breast feed while taking this drug without consulting physician.

Common adverse effects in italic, life-threatening effects underlined; generic names in bold; classifications in SMALL CAPS; Canadian drug name; Prototype drug