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Letter to the editor

Editor, – Regarding insulin and metformin schedules - indeed one size does not fit all. Dr Pat Phillips' excellent update 'Insulins in 2002' (Aust Prescr 2002;25:29-31) nicely highlights inter-individual insulin requirements (e.g. a predicted daily range of 39 to 78 units of insulin for a 78 kg man).

When metformin is factored into the equation, the considerations become even more complex, as when for example a patient has mild diabetes-related renal dysfunction and/or chronic low-grade hepatitis B, both of which are relative contraindications to the use of metformin.

I am also currently looking after a man in his 70s who is mildly overweight, with borderline urea and creatinine, chronic hepatitis B with a slightly raised GGT but normal ALT concentration. His insulin requirements exceed 100 units per day, but metformin is being withheld out of concern for potential adverse effects.

In view of the potential value of metformin with insulin, would Dr Phillips care to comment further on the nuances of this interesting combination of drugs?

Ross Philpot
Consultant Physician
Adelaide

Author's comments

Dr P. Phillips, the author of the article, comments:

Dr Philpot correctly points out the advantages of continuing metformin when starting insulin in patients with type 2 diabetes. Metformin has actions independent of insulin secretion (by reducing gluconeogenesis and insulin resistance) and it has benefits in controlling weight.

However, metformin can cause potentially life-threatening lactic acidosis in patients at risk of metformin accumulation (renal impairment), hypoxic challenges (respiratory or cardiac failure) or reduced lactate clearance (impaired liver function).

The first patient described by Dr Philpot had 'mild diabetes-related renal dysfunction and/or chronic low-grade hepatitis B'. If the patient had one relative contraindication (moderate renal impairment, GFR 30-60 mL/minute) our guidelines1 would recommend that low doses of metformin are appropriate (500-1000 mg/day). The situation should be reviewed regularly and metformin should be stopped if the patient were to develop an absolute contraindication.

In the second case it appears that the patient might have moderate renal impairment (GFR 30-60 mL/minute) but no functional liver impairment. A metformin dose of 500-1000 mg/day would seem appropriate and might reduce the necessary insulin dose and improve glycaemic control.

References

  1. Phillips P, Braddon J. Oral hypoglycaemics. When not to use what. Aust Fam Physician 2002;31:637-43.