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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(Glucophage®, Fortamet®, Riomet®) is an anti-diabetic drug from the biguanide class (its other members are the withdrawn agents phenformin and buformin).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Phenformin was withdrawn because of an increased risk of lactic acidosis (up to 60 cases per million patient-years).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 helps patients with polycystic ovary syndrome
From American Family Physician, 5/1/04 by Anne D. Walling

In polycystic ovary syndrome, the triad of anovulation, infertility, and hyperandrogenism manifests clinically as hirsutism, acne, and irregular menstruation. Ten percent of women of reproductive age are affected, and about 40 percent of them develop impaired glucose tolerance or overt type 2 diabetes by 40 years of age. Lord and colleagues studied the effectiveness of metformin in the treatment of polycystic ovary syndrome.

The authors searched databases of clinical trials, the Cochrane register of trials, and the reference sections of all of the identified trials to access relevant clinical studies. Two independent reviewers assessed each trial for quality and, if necessary, authors were asked to provide unpublished data or clarifying information. Of the 20 trials initially identified, 13 met the criteria for inclusion in this review.

Five trials included information on pregnancy and reported that metformin therapy did not have any advantage over placebo in rates of conception, although the combination of metformin and clomiphene was significantly more effective than therapy with clomiphene alone. This combination also was significantly more effective than clomiphene alone for inducing ovulation. Both treatments were significantly superior to placebo. Metformin therapy did not benefit body weight, body mass index, waist circumference, or waist-to-hip ratio. In the two trials that monitored blood pressure, metformin therapy was associated with significant reductions in systolic and diastolic blood pressures. In biochemical measures, metformin therapy was associated with significant reductions in fasting insulin concentrations and low-density lipoprotein concentrations. No evidence was found of any effect on total cholesterol level, triglycerides, or high-density lipoprotein concentrations.

The authors conclude that metformin therapy is effective for inducing ovulation in women with polycystic ovary syndrome, achieving results in 46 percent of patients (equivalent to a number needed to treat [NNT] of 4.4). When metformin was combined with clomiphene, 76 percent of treated women ovulated compared with 42 percent when clomiphene was used alone (NNT of 3.0). Although metformin has some beneficial effects on the metabolic syndrome, they are small. No net effect on weight loss is apparent. Gastrointestinal side effects can limit the use of metformin, but no serious long-term adverse effects are known. Metformin's effect on pregnancy is unclear, and it should not be used if any degree of renal impairment is present because of the danger of lactic acidosis.

Lord JM, et al. Metformin in polycystic ovary syndrome: systematic review and meta-analysis. BMJ October 25, 2003;327:951-6.

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