Metformin: the safest hypoglycaemic agent in chronic kidney disease?

Metformin is the first-line oral agent in the treatment of type 2 diabetes and has many established benefits, including the reduction of macrovascular complications of diabetes. Its prescription in patients with renal impairment is limited by concerns relating to the theoretical risk of lactic acidosis, a fear which is perpetuated by numerous case reports in which it is implicated. Critical review of this literature calls into question the validity of these claims, with metformin usually acting as an 'innocent bystander' in acutely unwell patients with conditions well recognised to precipitate lactic acidosis such as sepsis or hypovolaemia. In fact, the evidence supports the safe use of appropriate doses of metformin in patients with chronic stable renal impairment, and highlights the important possible greater risks of the alternatives, most notably severe hypoglycaemia in patients taking sulphonylureas and/or insulin and fluid retention in patients taking a thiazolidinedione. Other traditional contraindications to metformin use such as heart failure are also being re-evaluated, as the benefits of metformin in these patients are increasingly recognised. Physicians should weigh this evidence carefully before deciding to withdraw metformin therapy in their patients with stable chronic kidney disease.