The skin barrier makes drug penetration a major challenge in ensuring the efficacy of topical drug delivery. Penetration enhancer excipients aid in the absorbtion of an active pharmaceutical ingrediant (API) into the body via the skin. The ability of an excipient to aid in this absorbtion or solubility depends greatly on the chemical properites of the API. The rate at which a drug diffuses across the outermost layer of skin determines its overall rate of dermal penetration and permeation.