Surface Effects

Of the many possible dermatological targets mentioned above, the skin surface is clearly the easiest to access. Surface treatment begins at the fringe of cosmetic practice. Special cosmetics are available to hide unsightly blemishes and birthmarks. These lessen self-consciousness and are psychologically uplifting. Applying a protective layer over the skin is sometimes desirable. For example, zinc oxide pastes are used to create a barrier between an infant and its diaper that adsorbs irritants found in urine, ameliorating diaper rash. These same pastes literally block out the sun and at the same time hold in moisture, protecting the ski enthusiast from facial sun and windburns on the high slopes. Transparent films containing ultraviolet light-absorbing chemicals are also used as sunscreens. Lip balms and like products lay down occlusive (water-impermeable) films over the skin, preventing dehydration of the underlying stratum corneum and thereby allaying dry skin and chapping. The actions of calamine lotion and other products of the kind are limited to the skin's surface. The suspended matter in these purportedly binds urushiol, the hapten (allergen) found in poison ivy and oak. However, these may best benefit the patient by drying up secretions, relieving itchiness. In all these instances where the film itself is therapeutic, bioavailability has little meaning.

Bioavailability does matter with topical antiseptics and antibiotics, even though these also act mainly at the skin's surface. These anti-infectives are meant to stifle the growth of surface microflora, and thus formulations that penetrate into the cracks and fissures of the skin where the microorganisms reside are desirable. The extent to which the surface is sanitized then depends on uptake of the anti-infective by the microbes themselves. Slipshod formulation can result in a drug being entrapped in its film and inactivated. For instance, little to no activity is to be expected when a drug is placed in a vehicle in which it is highly insoluble. Ointment bases that contain salts of neomycin, polymyxin, and bacitracin are suspect in this regard in that hydrocarbon vehicles are extremely poor solvents for such drugs. A pharmacist should seek evidence that such formulations are effective before recommending them. Inunction (rubbing in) may release such drugs, the sebum on skin surface may dissolve a fraction of lipophilic drugs, and sweating may make the water-soluble drug available for absorption. Particularly, application of cosmetic products, insect repellents, sunscreens on the sites where topical drug applied, either before or after, may have profound effect on percutaneous absorption of drugs from dermatological products.

Deodorants are also targeted to the skin surface to keep microbial growth in check. Here, they slow or prevent rancidification of the secretions of apocrine glands found in and around the axillae (armpits) and the anogenital regions. Medicated soaps also belong to this family.

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