than retinyl esters but far less likely than topical tretinoin to cause retinoid dermatitis.16 It is also of interest that retinol has been shown to penetrate human skin more readily than retinyl palmitate, retinaldehyde, and retinoic acid, although retinol did not produce erythema in this study.17 Irritation potential with retinol is also concentration-dependent and can be minimized by delivering stabilized low-concentration retinol.18
Commercially available over the counter retinols differ vastly. Most retinols are formulated with concentrations of 0.04%–1% retinol, although few are labeled with this information. Intuitively, it might seem reasonable to assume that higher concentration retinols are more effective, but there are other formulation parameters that affect potency and efficacy. In a comparative study, retinol bioactivity as indicated by cellular retinoic acid binding protein II (CRABP II) expression, was greater with stabilized 0.1% retinol versus prestige brand 0.2% and 1.0% retinols.19 Delivery systems such microsponges and liposomes have been used to deliver retinol to the skin and have an effect on potency, efficacy and tolerability.20 Modern advances in formulation science and analytical stability techniques have enabled the
use of consumer-friendly
