Many drugs can be applied to the skin without entering the blood. How is this fact explained by the structure of the skin

The skin is made of epidermis and dermis layer. The blood vessel can be found in the dermis layer, a layer below the epidermis. To reach the blood, the drug has to pass through the epidermis.

Epidermis is a layer that made of keratinized squamous epithelial cell that located in the outer layer. The epidermis layer is strong and prevents movement of water, which makes them hard to be penetrated by drugs. If the drug doesn't have a good penetration, it will not reach the dermis, thus not reaching the blood vessel.