This review is quite limited—consult your physician or pharmacist for full details. Remember that drug names vary by country and manufacturer. Glucophage is a common brand name for metformin in the U.S.
Biquanide (it’s the only one in this class).
How does it work?
In short, metformin decreases glucose output by the liver. The liver produces glucose (sugar) either by breaking down glycogen stored there or by manufacturing glucose from smaller molecules and atoms. The liver then kicks the glucose into the bloodstream for use by other tissues. Insulin inhibits this function of the liver, thereby keeping blood sugar levels from getting too high. Metformin improves the effectiveness of insulin in suppressing sugar production. In other words, it works primarily by decreasing the liver’s production of glucose.
Physicians talk about metformin as an “insulin sensitizer,” primarily in the liver but also to a lesser extent in peripheral tissues such as fat tissue and muscle. It doesn’t work without insulin in the body.
Metformin typically lowers fasting blood sugar by about 20% and hemoglobin A1c by 1.5% (absolute decrease, not relative).
When used as the sole diabetic medication, metformin is associated with decreased risk of death and heart attack, compared to therapy with sulfonylureas, thiazolidinediones, alpha-glucosidase inhibitors, and meglitinides.
Not uncommonly, metformin leads to a bit of weight loss and improved cholesterol levels. Insulin and sulfonylurea therapy, on the other hand, typically lead to weight gain of 8–10 pounds (4 kg) on average.
Metformin works by itself, but can also be used in combination with most of the other diabetic medications. It’s usually taken 2–3 times daily.
Starting dose is typically 500 mg taken with the evening meal. The dose can be increased every week or two. If more than 500 mg/day is needed the second dose—500 mg—is usually given with breakfast. Usual effective maximum dose is around 2,000 mg daily.
Metallic taste, diarrhea, belly pain, loss of appetite. Possible impaired absorption of vitamin B12, leading to anemia. When used alone, it has very little risk of hypoglycemia. Rare: lactic acidosis.
Don’t use metformin if you have . . .
Impaired kidney function (keep reading), congestive heart failure of a degree that requires drug therapy (this is debatable), active liver disease, chronic alcohol abuse.
Regarding impaired kidney function: don’t use metformin if your eGFR (estimated glomerular function rate) is under 30 ml/min/1.73 m squared), and use only with extreme caution if eGFR drops below 45 while using metformin. Don’t start metformin if eGFR is between 30 and 45. Your doctor can calculate your eGFR and should do so annually if you take metformin.
Updated April 10, 2016