Which statement is true regarding skin thickness and drug absorption?

Which statement is true regarding skin thickness and drug absorption?

• A. Penetration is directly proportional to skin thickness
• B. Penetration is independent of skin thickness
• C. Penetration is inversely related to skin thickness
• D. Penetration increases with thickness unpredictably

Answer: (C)

As diffusion through skin is the main route for many topical drugs, the thickness of the skin barrier determines how far a drug must travel. For passive diffusion, the flux is driven by the concentration gradient but is inversely related to the barrier’s thickness—the longer the diffusion path, the slower the penetration. That means thicker skin presents more resistance and reduces absorption, while thinner skin allows quicker penetration. This is why sites with thicker stratum corneum, like palms or soles, show lower absorption than thinner areas. Exceptions exist when the formulation or enhancers alter permeability, but the general relationship is that penetration decreases as skin thickness increases.

As diffusion through skin is the main route for many topical drugs, the thickness of the skin barrier determines how far a drug must travel. For passive diffusion, the flux is driven by the concentration gradient but is inversely related to the barrier’s thickness—the longer the diffusion path, the slower the penetration. That means thicker skin presents more resistance and reduces absorption, while thinner skin allows quicker penetration. This is why sites with thicker stratum corneum, like palms or soles, show lower absorption than thinner areas. Exceptions exist when the formulation or enhancers alter permeability, but the general relationship is that penetration decreases as skin thickness increases.