At least 20 therapeutically important variables influence medication and chemical percutaneous absorption: Skin appendages as sub-anatomical pathways; skin application sites, population variability, skin surface conditions, skin health and integrity, substantivity and binding to different skin components, systemic distribution and systemic toxicity, stratum corneum exfoliation, washing-off and washing-in, rubbing/massaging, transfer to others, volatility, metabolic biotransformation/cutaneous, Photochemical change and photosensitivity, excretion pharmacokinetics, lateral distribution, and a chemical technique for assessing percutaneous absorption are all investigated.