Five studies (n=149) were included in the review. Three trials had a parallel design and two a crossover design.
Metformin treatment showed a small but statistically significant improvement in HbA1c (SMD -0.346, 95% CI -0.052 to -0.640), which was equivalent to a reduction of 0.27%. There was also a moderate reduction in insulin dose (SMD -0.422, 95% CI -0.148 to -0.735). Three studies also found improvements in total cholesterol (WMD -0.433, 95% CI -0.812 to -0.054). There was no significant change in weight.
No adverse events that required cessation of therapy were reported. There was no evidence of statistically significant heterogeneity in any of the analyses. There was also no evidence of publication bias.