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Metformin hydrochoride (Glucophage / Glucophage SR) is an anti-diabetes drug. It is particularly useful in treating diabetes in patients with insulin resistance and obesity.

Metformin can be taken alone, or if this is not effective, in combination with other drugs such as rosiglitazone (Avandia). Metformin and rosiglitazone are available in a combination tablet called Avandamet, manufactured by GlaxoSmithKline.

The way in which metformin works is uncertain, although it is thought to reduce the body’s rate of glucose production in the liver. It also reduces the rate of glucose absorption from the gut.

Metformin can reduce the insulin resistance that can occur as a side-effect of HIV treatment, notably with protease inhibitors. The drug can also improve perturbations of blood fat and sugar levels that can occur with HIV treatment, particularly by reducing triglyceride levels. These improvements can reduce the chance that a patient will suffer a stroke or heart attack.

Metformin also reduces fat accumulation around the internal organs. However, it can worsen fat loss from under the skin, a more worrying aspect of fat redistribution for most HIV-positive patients[1][2][3][4][5].

Side-effects of metformin include diarrhoea, stomach pain, nausea and vomiting. These can be reduced by taking the drug with or after food.

Metformin can also cause elevated levels of lactic acid in the blood, although this is usually only seen in patients with liver, heart or kidney problems. However, lactic acidosis is also a side-effect of some nucleoside reverse transcriptase inhibitors (NRTIs), notably ddI (didanosine, Videx / VidexEC) and d4T (stavudine, Zerit). Patients taking these drugs with metformin may be at an elevated risk of lactic acidosis: one case report has identified this interaction as causing fatal lactic acid elevations[6]. For more information on lactic acidosis, see Hyperlactataemia and lactic acidosis.

Metformin does not interact with any currently available protease inhibitors or non-nucleoside reverse transcriptase inhibitors (NNRTIs).