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Why am I taking metformin?

Asked by Marcus, Houston, Texas

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I was controlling my blood sugar with regular insulin injections, so why did my doctor add metformin during my last visit?

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Conditions Expert Dr. Otis Brawley Chief Medical Officer,
American Cancer Society

Expert answer

Type 2 diabetes mellitus is a disease that can initially show no symptoms. Eventually very high blood sugars cause symptoms of blurred vision, increased urination, and increased thirst.

The long-term effects of diabetes, especially uncontrolled diabetes, can include cardiovascular disease, heart attack and stroke, peripheral vascular disease, and kidney disease.

In type 2 diabetes, the cells of the muscles and organs of the body have difficulty bringing blood sugar, which is also called blood glucose, inside to use as fuel for metabolism.

Insulin's normal function is to help bring sugar into the cell, and the problem is these cells have what is called insulin resistance. The body's response to insulin resistance is to increase the amount of insulin secreted by the pancreas.

Over time, higher and higher amounts of insulin are secreted to overcome insulin resistance. Eventually the blood sugar levels rise higher than normal despite the high levels of circulating insulin.

Type 1 diabetes differs from type 2 diabetes in that type 1 is a disease in which the pancreas stops producing insulin. The initial treatment of mild type 2 diabetes mellitus is lifestyle intervention. This usually consists of diet modification, exercise and weight loss.

If this does not control blood sugars, metformin is usually the first drug prescribed. Metformin is an oral medication that is taken once or twice a day. It decreases sugar production in the liver and decreases blood sugar levels by increasing muscle and organ sensitivity to insulin.

The dose of metformin can be increased over a period of weeks to months until the desired effect is obtained or side effects become significant. Metformin at higher doses can cause gastrointestinal problems such as metallic taste, nausea, abdominal discomfort and diarrhea.

It is not uncommon that the dose has to be adjusted downward before getting the desired blood sugar result. Metformin is used as an initial therapy for mild to moderate type 2 diabetes; the initial treatment of moderate to severe diabetes mellitus is with insulin.

An example of a moderate to severe diabetic at diagnosis is a person with a hemoglobin A1C of 10% or greater, a fasting glucose of greater than 250 mg/dl (13.9 mmol/L), random blood sugars of greater than 300 mg/dL (16.7 mmol/L), or ketones in the urine.

Patients who are initially treated successfully with metformin commonly need additional therapy with time. For a physician with a large practice of diabetics, it is estimated that patients need something in addition to metformin at a rate of about 5% to 10% per year.

These patients who need additional therapy generally have hemoglobin A1C levels greater than 7%. Options for additional therapy are insulin or a sulfonylurea drug. Sulfonylureas have the convenience of being taken orally. They cause the pancreas to secrete even more insulin.

Metformin with insulin and metformin with sulfonylureas are synergistic: In either combination, the drugs work together. It makes sense to give a drug that overcomes resistance to insulin and at the same time increase the total amount of insulin in the patient. Many doctors prefer insulin as the second therapy as it is the most proven effective and the least expensive.

Insulin is definitely preferred for those with symptoms of a high blood sugar or a hemoglobin A1C greater than 8.5% while on metformin. Sulfonylurea drugs are associated with weight gain and the same low blood sugar risks associated with insulin therapy.

Newer, very expensive drugs are on the market and gaining popularity. Some doctors will advise long-term use of metformin and insulin together. Others prefer to discontinue the metformin after a while and use insulin only. Insulin-only therapy is associated with weight gain that is generally not seen with the combination of metformin and insulin.

I understand a patient's reluctance to start insulin as it is a sign that the disease is worsening. Insulin is injected under the skin with a needle and syringe. Some patients use insulin up to four times a day.

With some dedication and planning, one can use insulin and not have significant life restrictions. Good blood sugar control is crucial in avoiding so many of the complications of diabetes.

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