Answer:
Due to small size.
Explanation:
The topical drug pass through the skin layer because of the ability to pass through the epidermis of the skin. The size of particles of these topical drug are very small which can easily pass through the skin layer and added into the blood stream where they perform its function inside the body on the targeted region. If the size of these topical drugs are large so it can not enter inside the skin so that 's why we can say that small size is responsible for entering of topical drug.
