Metformin has been used as a first line agent in the management of type 2 diabetes for over 40 years. It is inexpensive, effective (twelve patients need to be treated for 10 years to prevent one diabetes-related endpoint),1 and in contrast to other drugs used to treat diabetes, does not usually cause hypoglycaemia or weight gain.2 It is generally well tolerated, although it can produce mild gastrointestinal effects that usually settle with time. Sales of another biguanide (phenformin) were suspended in the 1970s as a result of a high rate of lactic acidosis associated with its use. Naturally this raised concerns of a class effect and metformin has been implicated in a number of case reports and case series of lactic acidosis, particularly in people with kidney disease. …