Contraindications to use of metformin. Metformin may be useful in gestational diabetes.

Editor—Jones et al criticise current guidelines highlighting possible contraindications to the use of metformin as too vague and potentially leading to underuse in patients with type 2 diabetes.1 Although their desire for a “less ambiguous” approach seems sensible, their own guidelines still lack clarity. They note that any specific value of serum creatinine concentration chosen as a cut-off point for prescribing metformin will be arbitrary because of variations in muscle mass and protein turnover. Despite this they then select—for undefined reasons—a serum creatinine value of 150 μmol/l as the cut-off point in their guideline. They then say that caution should therefore be used in prescribing metformin for elderly patients. This is a vague statement, which could be interpreted as meaning that metformin should not be prescribed at all for elderly people, that specialist opinion should be sought or creatinine clearance calculated before it is prescribed, or that renal function or serum lactate concentration should be monitored after it is prescribed.2 In addition, the authors do not define elderly. Given that the growing majority of patients with type 2 diabetes are over 65 and that there is already evidence of undertreatment in such patients,3 it seems particularly desirable to be as clear as is possible about this age group if the full benefits of treatment are to be attained. Although the simple formulas that can be used to estimate creatinine clearance are not completely reliable,4 it may be preferable for prescription to be related to such a calculated estimate of clearance, which takes into account a patient's age, rather than the serum creatinine value alone.