Preadmission usage of metformin and mortality in COVID-19 patients including the post-discharge period

Background The effect of preadmission metformin usage (PMU) on the mortality of coronavirus disease-2019 (COVID-19) patients with diabetes is conflicting. Most studies have focused on in-hospital mortality; however, mortality after discharge also increases in COVID-19 patients. Aims Examining the effect of PMU on all-cause mortality, including the post-discharge period. Methods Patients with diabetes who were hospitalised in 2020 due to COVID-19 were included in the study. They were divided into two groups: those with a history of metformin use (MF( +)) and those without such history (MF( −)). Propensity score matching (PSM) was performed at a ratio of 1:1 for age and sex. COX regression analyses were used to demonstrate risk factors for mortality. Results We investigated 4103 patients hospitalised for COVID-19. After excluding those without diabetes or with chronic liver/kidney disease, we included the remaining 586 patients, constituting 293 women (50%) with an overall mean age of 66 ± 11.9 years. After PSM analysis, the in-hospital and post-discharge mortality rates were higher in the MF( −) group though not significantly different. However, overall mortality was higher in the MF( −) group (51 (42.5%) vs. 35 (29.2%), p = 0.031). For overall mortality, the adjusted HR was 0.585 (95% CI: 0.371 − 0.920, p = 0.020) in the MF( +) group. Conclusion PMU is associated with reducing all-cause mortality. This effect starts from the in-hospital period and becomes more significant with the post-discharge period. The main limitations were the inability to evaluate the compliance with metformin and the effects of other medications due to retrospective nature.