To determine the incidence of lactic acidosis in community-based patients with type 2 diabetes, with special reference to metformin therapy.

Substudy within a longitudinal observational study, the Fremantle Diabetes Study (FDS).

1279 patients from a postcode-defined population of 120 097 people in Western Australia.

Confirmed hospitalisation with lactic acidosis identified through the WA Data Linkage System during two periods: (1) from study entry, between 1993 and 1996, and study close in November 2001; and (2) from study entry to 30 June 2006.

At entry, 33.3% of patients were metformin-treated, and 23.1% of these had one or more contraindications to metformin (55.1% and 38.0%, respectively, after 5 years’ follow-up). Five confirmed cases of lactic acidosis were identified during 12 466 patient-years of observation; all had at least one other potential cause, such as cardiogenic shock or renal failure. From study entry to close, the incidence was 0/100 000 patient-years in both metformin-treated and non-metformin-treated patients. Between study entry and 30 June 2006, incidence was 57/100 000 patient-years (95% CI, 12–168) in metformin-treated patients and 28/100 000 patient-years (95% CI, 3–100) in the non-metformin-treated group, an incidence rate difference of – 30 (– 105 to 46) (P = 0.4).

The incidence of lactic acidosis in patients with type 2 diabetes is low but increases with age and duration of diabetes, as cardiovascular and renal causes become more prevalent. Metformin does not increase the risk of lactic acidosis, even when other recognised precipitants are present.