Metformin use & asthma outcomes in patients with concurrent diabetes & bronchial asthma: A systematic review & meta-analysis.

Background & objectives Limited data exist on the effects of metformin on asthma outcomes in patients having type 2 diabetes (T2D) along with asthma. This meta-analysis seeks to determine whether metformin influences asthma outcomes in these patients. Methods Studies involving adults with T2D and asthma, using metformin in the intervention group and other glucose-lowering drugs in the control group, were systematically searched through databases. The primary outcome was the adjusted risk of asthma exacerbations in metformin users (MU) vs. non-metformin users (non-MU). Other outcomes included asthma-related emergency room visits, hospitalisations, and the need for rescue steroids. Meta-analysis was conducted using RevMan with random-effects models. Outcomes were reported as hazard ratios (HRs) with 95 per cent confidence intervals (95% CI). Results Six retrospective cohort studies with moderate overall risk of bias, involving 317,905 patients, were included. The risk of asthma exacerbation was comparable in MU and non-MU (HR 0.95, 95% CI: 0.86, 1.04, P=0.27). The risk of asthma-related emergency room visits appeared lower in MU than in non-MU, though the difference did not gain statistical significance (HR 0.63, 95% CI: 0.39, 1.01, P=0.06). The two groups also had statistically identical risks of asthma-related hospitalisations (HR 0.76, 95% CI: 0.54, 1.07, P=0.12) and use of rescue steroids (HR 0.95, 95% CI: 0.87, 1.03, P=0.24). However, after excluding a study from Taiwan (n=115486), a significantly lower risk of asthma exacerbation (HR 0.89, 95% CI: 0.83-0.97, P=0.005) and asthma-related hospitalisations (HR 0.68, 95% CI: 0.49-0.93, P=0.02) was observed in MU. Interpretation & conclusions Metformin therapy for T2D does not influence asthma outcomes; however, certain outcomes appear to improve when a Taiwanese study is excluded. Such results must be considered cautiously, as the included studies are observational and have a moderate risk of bias. Randomised controlled trials with diverse global representation are essential for clarifying the relationship between metformin use and asthma outcomes.