**7.1. Skin**

The superficial layer of the skin, stratum corneum is particularly impermeable to most drugs. The skin permeability for the drugs is enhanced by using a co-solvent system such as ethanol/ water which increases drug partition into the skin. The lipid domains of the buccal and nasal mucosa also restrict drug entry and the drugs which permeate are able to do so through passive diffusion using the hydrophilic trans-cellular spaces and direct permeation through the membrane.
