For how we live.
A 19-trial review of peptides prints two different wrinkle effect sizes in its own abstract. A vitamin C trial ran four weeks against retinol, with nothing inert to compare either of them against.
Tretinoin beats its own vehicle in randomised trials — though the vehicle does much of the work. In the few head-to-head trials against over-the-counter retinol, both arms improved and tretinoin irritated more.
Collagen, flavanols and polyphenols beat their comparators on skin elasticity and sun tolerance. The pooled estimates carry heterogeneity of 95% to 98%, and some comparators were low doses of the same ingredient.