New interventions are rapidly progressing into clinical practice through the preclinical study stage. The animal experiments should be comprehensive and critically evaluated for their pivotal role in this process. To integrate the evidence of animal studies by systematic review throws light on intervention outcomes and could decrease risk of participants in human trials. The methodological weakness of animal studies is easy to highlight but robust methods to synthesize evidence is essential. Here, we discuss some problems in systematic review of animal experiments and whether animal models of diverse species contribute to bias of meta-analysis conclusions.