Metformin, classified as a biguanide, doesn’t enhance insulin secretion, so it won’t predispose the patient to hypoglycemia or cause the weight gain associated with other glucose-lowering drugs. Metformin lowers blood glucose levels and improves glucose tolerance by decreasing glucose production in the liver and increasing the muscles’ use of insulin. It also decreases intestinal absorption of glucose.Metformin increases the binding of insulin to its cellular receptor sites and decreases the potential for insulin resistance. The drug enhances the uptake of glucose by the muscles and adipose tissue. During the day, especially after lunch, the drug lowers postprandial blood glucose levels. This phenomenon may be attributed to delayed glucose absorption or to the time required for metformin to be absorbed and accumulate in the tissues.

Taking the drug also reduces levels of serum triglycerides, total cholesterol, LDL cholesterol and increases levels of HDL cholesterol.

The initial dose of metformin is 500 mg twice a day, with morning and evening meals. A physiciangradually increases the dose by 500 mg per week until the patient’s fasting blood glucose and glycosylated hemoglobin levels are as close to normal as possible. The total recommended dosage is 2,500 mg per day in three doses. Metformin can be used alone or with sulfonylurea therapy.

Indications and Contraindications

Patients with Type 2 diabetes whose disease can’t be managed satisfactorily by diet and exercise alone take metformin to reduce hyperglycemia. If metformin and diet therapy are ineffective, a physician may add a sulfonylurea.

Before your patient begins metformin therapy, assess her kidney function and verify that her serum creatinine levels are below 1.5 mg/dl to prevent lactic acidosis. If they’re above 1.5 mg/dl, her physician will substitute another drug. Instruct your patient to contact her physician immediately if she experiences difficulty breathing, muscle aches, fatigue, unusual sleepiness, or nonspecific symptoms, which may indicate lactic acidosis.

Metformin is contraindicated for patients with kidney dysfunction, cardiopulmonary disease, infection, hypersensitivity to the drug, or ketosis. Patients undergoing radiologic tests such as excretory urography, angiography, and scans that use parenteral iodinated contrast media shouldn’t receive metformin within 48 hours of the test. The contrast media can predispose a patient to acute kidney failure and an accumulation of metformin in the kidneys, which can lead to lactic acidosis.

A physician should stop metformin therapy before surgery if food and fluid intake is restricted. The patient shouldn’t resume therapy until kidney function has returned to her base­line and she’s eating meals.

Usually, insulin and metformin aren’t used simultaneously. If a patient needs insulin, metform in should be discontinued.

Adverse Effects and Interactions

The most common adverse effects of metformin are GI signs and symptoms such as diarrhea, anorexia, nausea, bloating, and a metallic taste in the mouth. Tell your patient that she can prevent GI irritation by taking metformin with food, which also helps to delay rapid drug absorption. Patients who have difficulty swallowing may crush metformin and mix it with food or liquids. If diarrhea or vomiting is severe, your patient’s physician may stop metformin therapy temporarily.

Patients who don’t get enough vitamin B 12 or calcium may develop asymptomatic vitamin B 12 deficiency when taking metformin because the drug interferes with the absorption of the vitamin. Taking vitamin B 12 supplements or discontinuing metformin therapy rapidly reverses the deficiency.

If your patient is taking a drug for a bacterial infection, angina, thyroid disease, coughing, asthma, a skin condition, or anxiety, it may alter the effects of her metformin therapy. For example, many calcium channel blockers and corticosteroids can decrease the effects of metformin and may seriously undermine blood glucose control.