ABSORPTION OF DRUGS THROUGH THE SKIN
Most drugs that have been incorporated into creams or ointments are applied to the skin for their local effect. The diffusion rate of a drug through the skin is largely determined by the compound’s lipid–water partition coefficient. However, the stratum corneum, or outer layer of the epidermis, forms a barrier against the rapid penetration of most drugs. This is due in large part to the relatively close-packed cellular arrangement and decreased amount of lipid in these cells. Thus, even highly lipid-soluble compounds will be absorbed much more slowly through the skin than from other sites. The dermis, on the other hand, is well supplied with blood and lymph capillaries and therefore is permeable to both lipid-soluble and water-soluble compounds. If penetration of the skin by lipid-insoluble compounds does occur, it is probably accomplished by diffusion through the hair follicles, sweat glands, or sebaceous glands.