CRD Summary: This review assessed the safety of metformin for people with diabetes and heart failure. The authors concluded that although metformin may be safe, further studies are needed to provide conclusive evidence. There were methodological limitations to the review, but these conclusions are suitably conservative given the limited evidence presented.

CRD Commentary: The inclusion criteria for this review were only partially stated: criteria for study design and outcomes of interest were vague. Searching was limited to two databases and there were no attempts to identify unpublished studies. It is possible that studies were missed and this could have affected the results of the review. The methods of the review were not described, thus it is not possible to comment on any likelihood of reviewer bias, or the possible introduction of errors into the process of the review. The quality of the included studies does not appear to have been systematically assessed, although the authors commented on several limitations of the included studies, particularly the retrospective studies, in which metformin use might have been avoided in patients with more severe CHF. The authors acknowledged that the information from the review is limited by the types of studies available for inclusion. Cohort studies are open to selection bias, and other factors, that make it difficult to establish a cause and effect relationship.  The authors’ conclusions are suitably conservative given the limited evidence presented.