Effect of metformin use on clinical outcomes and serum urate in gout patients with diabetes mellitus: a retrospective cohort study

Objective Gout and diabetes mellitus type 2 (DM) frequently co-exist. The pharmacological effects of metformin may include anti-inflammatory and urate lowering effects. The objective of this study was to test these effects in patients with gout starting uric acid lowering treatment (ULT) in secondary care. Methods Retrospective cohort study including patients with gout and DM starting ULT. Differences in the incidence density of gout flares, proportion of patients reaching target sUA in the first six months after starting ULT, and difference in mean allopurinol dose at sUA target were compared between users of metformin and users of other or no anti-diabetic drugs (control group). Correction for confounding was applied. Results A total of 307 patients were included, of whom 160 (52.1%) used metformin. The incidence of flares was 1.61 and 1.70 in the first six months for respectively the metformin group and control group. The incidence rate ratio for gout flares was not significant (0.95, 95% CI 0.78 to 1.14). At six months, 62.8% and 54.9% reached target sUA in the metformin and control group respectively, corrected odds ratio of 1.09 (95% CI 0.66 to 1.80). There was no difference in mean allopurinol dose at sUA target 266 mg for metformin users and 236 mg for the control group, difference 30 mg (95% CI − 4.7 to 65.5). Conclusions In conclusion we could not confirm a clinically relevant anti-inflammatory or urate lowering effect of metformin in patients starting ULT treatment and receiving usual care flare prophylaxis.