Collagen peptides are broken-down fragments of collagen, the most abundant protein in the body.
The evidence base here is 4 meta-analyses of randomized trials plus 3 individual randomized trials.
The three meta-analyses that report participant totals cover 1,721 (skin), 870 (knee joint), and 768 (training) participants.
In those studies collagen peptides are associated with improved skin hydration and elasticity, with improved knee pain and function scores in adults with knee osteoarthritis, and with small gains in fat-free mass and strength when combined with training.
Skin is the strongest signal. Joint findings conflict between meta-analyses, and the musculoskeletal effects are small with low GRADE certainty.
Collagen was well tolerated across these trials, and no drug interactions are documented in these sources.