Definition
A systematic review answers a focused research question by identifying, appraising and synthesising all relevant studies using predefined, reproducible methods. Reviewers register their plan in advance, often on PROSPERO, search multiple databases, screen studies against eligibility criteria, assess risk of bias and report following the PRISMA guidelines. When studies are similar enough, a meta-analysis statistically pools their results to give an overall estimate. Systematic reviews of randomised trials are generally ranked highest in evidence hierarchies, but their conclusions are only as strong as the included studies. For many research peptides, systematic reviews find mostly preclinical evidence, small sample sizes and limited independent replication.