Contraindications to metformin therapy in patients with Type 2 diabetes - a population-based study of adherence to prescribing guidelines

Aims To define the number of people in Tayside, Scotland (population 349 30 3) with Type 2 diabetes who use metformin, the incidence of contraindicatio ns to its continued use in these people and the proportion that discontinue d metformin treatment following the development of a contraindication. Methods Retrospective cohort study of the incidence of contraindications to metformin in all patients with Type 2 diabetes using metformin from Januar y 1993 to June 1995. The contraindications of acute myocardial infarction, cardiac failure, renal impairment and chronic liver disease were identified by: the regional diabetes information system, biochemistry database and ho spital admissions database and a database of all encashed community prescri ptions. Results One thousand eight hundred and forty seven subjects (26.3% of those with Type 2 diabetes) redeemed prescriptions for metformin. Of these, 3.5% were admitted with an acute myocardial infarction (71 episodes); 4.2% were admitted with cardiac failure (114 episodes); 21.0% received metformin and loop diuretics for cardiac failure concurrently; 4.8% developed renal impa irment; and 2.8% developed chronic liver disease. The development of contra indications rarely resulted in discontinuation of metformin, for example on ly 17.5% and 25% stopped metformin after admission with acute myocardial in farction and development of renal impairment, respectively. In total, 24.5% of subjects receiving metformin, 6.4% of all people with Type 2 diabetes, had contraindications to its use. There was one episode of lactic acidosis in 4600 patient years. Conclusions This population-based study shows that 24.5% of patients prescr ibed metformin have contraindications to its use. Development of contraindi cations rarely results in discontinuation of metformin therapy. Despite thi s, lactic acidosis remains rare.