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EDITORIALS: Munir Pirmohamed and Robin E Ferner Monitoring drug treatment BMJ 2003; 327: 1179-1181 [Full text]

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Monitoring Metformin Treatment

Paul S Johnstone, Mike Tomson   (11 December 2003)

Monitoring Metformin Treatment 11 December 2003
Paul S Johnstone, GP Registrar Belgrave Medical Centre, Sheffield S2 4UJ, Mike Tomson Send response to journal: Re: Monitoring Metformin Treatment	Pirmohamed and Ferner's editorial on "Monitoring Drug Treatment"(1) proved very timely with respect to a debate we are currently having within our Practice about monitoring of patients on Metformin. This relates to whether we should be checking annual B12 levels in these patients and what to do once a low level is discovered. The BNF warns that the side effects of Metformin include reduced absorption of vitamin B12. The manufacturer's Summary of Product Characteristics for Metformin recommends checking B12 levels annually for patients on continuous therapy. It has been estimated that between 10-30% of those on Metformin will develop some degree of B12 deficiency (2). There is the potential for a neuropathy due to B12 deficiency to be mistakenly attributed to a diabetic neuropathy. The manufacturer, Alpharma, in correspondence with the Practice, could not provide any specific references regarding treatment of low B12 levels in those on Metformin. They suggested either using B12 injections or reviewing the Metformin dose. Our local Diabetic clinic does not regularly screen B12 levels in their patients on Metformin and when asked, had no knowledge of any units that do. This appears to be a good example of product information providing advice regarding monitoring of a drug but there being no uniform response to this advice in practice. We have been unable to find evidence for the most appropriate action when a low B12 level is found. After excluding pernicious anaemia, do we stop or reduce the dose of Metformin thereby upsetting the patient's diabetic control or do we commence B12 injections? Could we be using oral vitamin B12 replacement effectively in this situation or possibly even using calcium supplements (3)? We have monitored B12 levels in our patients on Metformin annually for 2 years and a search shows that 7.9% are on B12 injections having been found to have a 'deficiency'. Amongst our practice population not on Metformin, i.e. the unscreened majority of the Practice, the prevalence of B12 'deficiency' is 0.9%. Does this level of change imply we are helping our patients more than the local hospital or that we are medicalising the normal? Should a manufacturer who recommends monitoring tests have a responsibility to ensure research is conducted on the best way to manage the changes that are then found? 1) Pirmohamed M, Ferner R. Monitoring drug treatment. BMJ 2003;327:1179-81 2) Tompkin GH, Haddon DR, Weaver JA, et al. Vitamin B12 status of patients on long-term metformin therapy. BMJ. 1971;2:685-687 3) Bauman WA, Shaw S, Jayatilleke E, Spungen AM, Herbert V. Increased intake of calcium reverses vitamin B12 malabsorption induced by metformin. Diabetes Care. 2000 Sep;23(9):1227-31 Competing interests: None declared