Which skin layer do transdermal lidocaine patches penetrate?

• A. Dermis
• B. Stratum corneum
• C. Epidermis
• D. Hypodermis

Answer: (B)

Transdermal drug delivery relies on crossing the skin’s outer barrier. That barrier is the stratum corneum, the outermost layer of the epidermis, composed of dead keratinocytes embedded in a lipid-rich matrix. For lidocaine from a patch to reach systemic circulation, it must diffuse through this layer. Once it gets past the stratum corneum, it moves through the deeper epidermal layers and into the dermis where it can access blood vessels. So, the layer that a transdermal lidocaine patch must penetrate first and most critically is the stratum corneum. The deeper tissues (epidermis proper, dermis, hypodermis) come after this initial barrier, but the primary barrier to penetration is the stratum corneum.