Level of Evidence
A classification of the quality and reliability of clinical evidence supporting a treatment recommendation. Level I evidence (systematic reviews of RCTs) is the strongest, while Level V (expert opinion) is the weakest. Most research peptides have only Level IV-V evidence from preclinical or observational data.
Technical Context
Common classification systems: Oxford Centre for Evidence-Based Medicine (OCEBM) levels 1-5, where Level 1 = systematic review of RCTs or single large RCT with narrow CI, Level 2 = smaller RCTs or large observational studies, Level 3 = case-control/cohort studies, Level 4 = case series/case reports, Level 5 = expert opinion. GRADE system (used by WHO, NICE, and Cochrane) takes a different approach, starting with study design (RCTs start at high, observational studies start at low) and adjusting up or down based on quality factors. For clinical guideline recommendations: Level 1 evidence + strong clinical benefit → strong recommendation (Grade A); lower evidence levels → weaker recommendations (Grade B, C, D). Most research peptides have only Level 4-5 evidence (case reports, expert opinion, extrapolation from preclinical data), while approved peptide drugs are supported by Level 1-2 evidence from clinical trial programmes.