Metformin chemical structure
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Metformin

Metformin (Glucophage®, Fortamet®, Riomet®) is an anti-diabetic drug from the biguanide class (its other members are the withdrawn agents phenformin and buformin). more...

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Uses

The main use for metformin is for the treatment of diabetes mellitus, especially when it is concomitant with obesity and insulin resistance.

It is also being used increasingly in polycystic ovary syndrome (PCOS) and non-alcoholic steatohepatitis, two other diseases that feature insulin resistance; these indications are still considered experimental.

Metformin is the only anti-diabetic drug that has been proven to reduce the complications of diabetes, as evidenced in a large study of overweight patients with diabetes (UKPDS 1998).

Metformin is often prescribed to type 2 diabetes patients in combination with rosiglitazone maleate. This drug actively reduces insulin resistance, complementing the action of the metformin. In 2002, the two drugs were combined into a single product, Avandamet, marketed by GlaxoSmithKline. . In 2005, all current stock of Avandamet was seized by the FDA and removed from the market. This was due to problems at the manufacturing plants, not to any medical issues resulting from the drugs use. The drug pair continued to be prescribed separately in the absence of Avandamet itself, which was readily available by the end of that year.

Mechanism of action

Despite its therapeutic benefits, the mechanism of action of metformin is uncertain. Its mode of action appears to be reduction of hepatic gluconeogenesis; the "average" person with type 2 diabetes has three times the normal rate of gluconeogenesis. Metformin treatment reduces this by one third to two thirds. It is has been shown that metformin also decreases intestinal absorption of glucose. A third mechanism is that metformin improves insulin sensitivity by increasing peripheral glucose uptake and utilization. Zhou et al (2001) showed that metformin stimulates the hepatic enzyme AMP-activated protein kinase.

Side-effects

The most serious side effect of metformin is lactic acidosis. However, this complication is rare if the contra-indications are followed, as it seems limited to those with impaired liver and/or kidney function.

Phenformin was withdrawn because of an increased risk of lactic acidosis (up to 60 cases per million patient-years). In recent studies it was revealed that, as long as it is not prescribed to patients who are at risk, metformin is much safer, and the risk of lactic acidosis approximates that of people who are not on the medication (Salpeter SR et al 2003).

The most common side effect of metformin is gastrointestinal upset. This includes diarrhea, cramps, nausea and vomiting. In a clinical trial of 286 subjects, 53.2% of the 141 who were given Metformin IR (as opposed to placebo) reported diarrhea, and 25.5% reported nausea/vomiting (source: Drug Facts & Comparisons 2005).

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Diabetes drug prescribed when contraindicated - metformin
From Nutrition Health Review, 3/22/03

Some doctors prescribe metformin, the most widely used pharmaceutical treatment for patients with type 2 diabetes and with congestive heart failure, kidney dysfunction, or both, despite clear warnings on drug packaging not to do so, a new study indicates.

At the University of North Carolina at Chapel Hill, it was noted that 22 of 100 randomly selected patients who were receiving metformin had one or both of the contraindications.

"Our findings are consistent with several recent studies in Europe that documented similar rates of inappropriate metformin prescribing," said Dr. Cheryl Horlen, who conducted the research as a pharmacy resident at the University. "We are concerned that these patients may be at risk of a serious medical condition known as lactic acidosis, which can be life-threatening."

In the first 14 months after its release in the U.S., the Food and Drug Administration (FDA) received reports of 47 confirmed cases of lactic acidosis associated with metformin, she said. The mortality rate was 42 percent. In the absence of contraindications, the drug is considered safe and effective.

A report on the study appears as a letter in The Journal of the American Medical Association (May 15, 2002).

"We performed a retrospective review of hospital charts of patients receiving metformin through the outpatient pharmacy at an academic medical center," Dr. Horlen said. "We found 241 patients who had two or more prescriptions for metformin within a 9-month period and then randomly selected 100 of those for the chart review."

Fourteen of the patients had congestive heart failure only, five had kidney dysfunction, and three had both. The mean age of patients was 60 years; half were women and half were black.

"Only two patients had documentation in their medical records that providers had considered the metformin contraindications," she said. "Because our assessment of the prevalence of contraindications to metformin use relied on a chart review, it might have underestimated the frequency of contraindications. It also was difficult to determine whether providers were aware they were prescribing metformin against what is called a black box warning, which is specific warning with a black border around it to attract attention."

Physicians and other health care providers should think about improving documentation of the risk of lactic acidosis and should offer counseling for patients who receive the drug, she said. Considering the new study and the European results, there is no reason to believe that metformin-prescribing practices differ much in other parts of the U.S.

"Although this was a relatively small study, the fact that almost one in four prescriptions for metformin were being prescribed against the black box warning may be significant," said Dr. Russell Rothman, a Robert Wood Johnson Clinical Scholar at the University. "With more than 25 million prescriptions of metformin written a year, the potential for adverse effects needs to be taken very seriously. More research will need to be performed to ensure that patients are receiving optimal care."

COPYRIGHT 2003 Vegetus Publications
COPYRIGHT 2003 Gale Group

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