Percutaneous permeation modifiers: enhancement versus retardation

Background: The use of permeation enhancers to compromise the barrier properties of skin has been ongoing for decades. However, toxicity associated with certain xenobiotics has led to the development of permeation retardants. Since both enhancers and retardants modify the surface layer of the skin, they can be collectively referred to as penetration modifiers. Objective: This review attempts to outline a comparison of two types of penetration modifiers: enhancers and retardants. Methods: in addition to reports of enhancement and retardation by modifiers, we also provide evidence as to why we should group these compounds together, since we have found that retardants can become enhancers in different formulation environments. Conclusion: Since modifiers influence drug delivery, further exploration of these compounds is required to understand their modifying action on the properties of skin.