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(Glucophage®, Fortamet®, Riomet®) is an anti-diabetic drug from the biguanide class (its other members are the withdrawn agents phenformin and buformin).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Phenformin was withdrawn because of an increased risk of lactic acidosis (up to 60 cases per million patient-years).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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Metformin-induced lactic acidosis extremely rare
From Journal of Family Practice, 4/1/04

Salpeter SR, Greyber E, Pasternak GA, Salpeter EE. Risk of fatal and nonfatal lactic acidosis with metformin use in type 2 diabetes mellitus. Arch Intern Med 2003; 163:2594-2602.

* CLINICAL QUESTION

What is the risk of lactic acidosis accompanying metformin therapy for patients with type 2 diabetes?

* BOTTOM LINE

The link between metformin and lactic acidosis, when used as prescribed, is tenuous. The bigger question is whether lactic acidosis risk truly increases when we relax criteria and give it to patients previously forbidden to take it. (LOE=1a)

* STUDY DESIGN

Systematic review

* SETTING

Outpatient (any)

* SYNOPSIS

It's tricky to try to prove the nonexistence of a phenomenon. The adage holds: Absence of proof is not proof of absence. So, how much absence of proof do we need?

The authors of this study combined the results of all randomized controlled trials and observational studies to determine the risk of lactic acidosis with metformin. The literature search was thorough and included unpublished data. Two independent reviewers evaluated articles for inclusion. The methodologic quality of the studies was evaluated using modified quality criteria. Of the 194 studies in the analysis, 126 were randomized controlled studies and 68 were observational research. More than 18,000 participants in these studies received metformin for an average 2.1 years (36,893 patient-years).

There were no cases of lactic acidosis in the metformin-treated group or in the comparison group. Not surprising, since patients with risk factors for lactic acidosis were undoubtedly not enrolled in any of the studies, and monitoring was more intense than in typical practice. Population studies estimate the rate of lactic acidosis to be between 2 and 9 cases per 100,000 patient-years (which is also the rate of lactic acidosis in patients with diabetes not receiving metformin). Using these numbers, 1 to 3 cases of lactic acidosis would have been expected. Several studies evaluated lactic acid levels in metformin-treated patients, finding no difference in baseline lactic acid levels compared with those not treated with metformin.

COPYRIGHT 2004 Dowden Health Media, Inc.
COPYRIGHT 2004 Gale Group

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