[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Pharmacology](https://durgical.com/en/pharmacology/) › Dermatological pharmacology: skin, vehicles and light # Dermatological pharmacology: skin, vehicles and light How drugs get through the skin, why the cream or ointment matters, and how light, sunscreens, itch treatments and keratolytics work. Updated Nov 12, 2026 On this page - The basics Key points - Skin structure and percutaneous absorption - Vehicles and occlusion - Sunscreens - Treating pruritus - Keratolytics for thick, scaly skin - Wound healing and scars - Memory aid - Example drugs - Treatment Light as therapy: PUVA, photopheresis and photodynamic therapy - Study Test yourself - Connections Connections The skin is both a target and a barrier. Its outer dead layer, the stratum corneum, decides how much of a topical drug gets in, and the vehicle (ointment, cream, gel, lotion) changes that a great deal. Dermatology also uses light as a drug partner (PUVA, photopheresis, photodynamic therapy), protects against light with sunscreens, and treats itch, thick scale and scars. ## Key points - The stratum corneum is the rate-limiting barrier; small molecules with both oil and water solubility cross it best. - Hydrating the skin with occlusive ointments or wraps increases absorption, which helps the drug but also raises the risk of local and systemic side effects. - PUVA combines a psoralen with UVA light; photopheresis treats a patient's own white cells with the same pair outside the body; photodynamic therapy uses a photosensitizer, visible light and oxygen. - SPF measures protection mainly against UVB (sunburn); broad-spectrum sunscreens also cover UVA. - Itch is treated by its cause: skin-origin itch with emollients, steroids and antihistamines; nerve-origin itch with gabapentinoids or topical anaesthetics. - Keratolytics (salicylic acid, urea, alpha-hydroxy acids, sulfur) loosen thick scale; salicylic acid on large areas of a child's skin can cause salicylate poisoning. ## Skin structure and percutaneous absorption Drugs cross the skin mainly through the winding spaces between the dead cells of the stratum corneum; passage through cells or hair follicles matters much less. The best topical drugs are small, soluble in both oil and water, and leave their vehicle readily for the skin. Charged and very polar molecules hardly cross intact skin. Absorption varies between body sites and with the vehicle, so the same drug can act very differently on the palm and on the face. Transdermal patches avoid first-pass metabolism in the liver, although skin enzymes can still metabolize some drugs. ## Vehicles and occlusion - Ointments are greasy and occlusive: they hydrate the stratum corneum and so increase absorption. - Creams, gels, lotions, solutions, foams and patches each trade penetration against comfort and the body site to be treated. - Newer delivery systems are designed to improve penetration or to release the drug slowly. - Occlusion with plastic film, gloves or a shower cap boosts absorption further but can cause folliculitis, maceration and more systemic absorption. - Topical glucocorticoids, ranked in classes by potency, show the trade-off: their adverse effects include skin atrophy, striae, telangiectasia, purpura and acne-like eruptions, and fluorinated steroids are avoided on the face. ## Light as therapy: PUVA, photopheresis and photodynamic therapy Psoralens such as methoxsalen are plant-derived furocoumarins that slip into DNA; when UVA light hits them they bind DNA bases and produce reactive oxygen species. PUVA is used for psoriasis and vitiligo; its risks are sunburn-like phototoxicity, eye damage without UVA-blocking glasses and a skin cancer risk that grows with the number of treatments. In extracorporeal photopheresis, the patient's blood white cells are separated, exposed to methoxsalen and UVA outside the body and returned; the treated lymphocytes die over the following days. It is used for cutaneous T-cell lymphoma and, off label, graft-versus-host disease. Photodynamic therapy applies a prodrug, aminolevulinic acid or its methyl ester, which cells turn into protoporphyrin IX; light then makes it produce reactive oxygen that kills the cells. It treats actinic keratoses and some superficial skin cancers. ## Sunscreens Sunscreens are one part of sun protection, together with shade, avoiding the midday sun, clothing, hats and sunglasses. Organic (chemical) filters absorb UV light and release it as heat; inorganic (physical) filters, zinc oxide and titanium dioxide, scatter and reflect it and cover a broad range. The sun protection factor compares the light needed to cause sunburn with and without the product, so it reflects UVB protection; a broad-spectrum label means UVA is covered too. FDA advice is a broad-spectrum product of SPF 15 or higher, reapplied at least every two hours and more often with sweating or swimming; water-resistant claims are limited to a stated time in water. Sunscreen is not recommended for babies under six months, who are kept out of the sun instead. ## Treating pruritus - Itch from the skin (eczema, xerosis, urticaria): emollients to repair the barrier, topical steroids and calcineurin or PDE4 inhibitors, H1 antihistamines, cooling agents such as menthol, capsaicin, phototherapy and IL-31 inhibitors. - Itch from diseased nerves: gabapentin, pregabalin, carbamazepine or topical anaesthetics. - Itch arising in the nervous system without nerve damage: opioid antagonists such as naltrexone, thalidomide and some antidepressants. - Psychogenic itch: anxiolytics, antidepressants or antipsychotics, together with treating the underlying condition. - Itch can signal internal disease (chronic kidney failure, liver and biliary disease, cancers), so the cause is sought before treating the symptom. ## Keratolytics for thick, scaly skin Keratolytics thin hyperkeratotic skin by loosening the bonds between corneocytes, adding water to the stratum corneum or speeding shedding. Salicylic acid dissolves the intercellular cement and is used for psoriasis scale, seborrhoeic dermatitis and warts; on large areas, especially in children or in kidney or liver disease, enough is absorbed to cause salicylism. Urea softens and hydrates the skin at low strength and dissolves keratin at high strength, for calluses and thick nails. Alpha-hydroxy acids (glycolic, lactic) thin the stratum corneum and increase sun sensitivity. Sulfur and propylene glycol are older keratolytics. ## Wound healing and scars Normal healing ends in a scar: fibroblasts lay down collagen without hair follicles or glands, in a disordered pattern that is weaker than normal skin. Mechanical tension drives this scarring, and experimental drugs that block tension signalling (such as verteporfin, used as a light-activated drug in the eye, and focal adhesion kinase inhibitors) have produced more skin-like healing in research. ## Memory aid Ointment is the Oil-tight lid: more Occlusion, more absorption, more side effects. ## Test yourself Think of the answer, then tap the question to see it. ## Which layer of the skin limits the absorption of topical drugs? The stratum corneum, the outer layer of dead cells; drugs cross it mainly through the spaces between the cells. ## Why does an ointment usually deliver more drug than a cream of the same strength? It is occlusive: it traps water in the stratum corneum, and hydrated skin lets more drug through. ## What does SPF measure, and what does broad spectrum add? SPF reflects protection against sunburn, which is mainly UVB; a broad-spectrum label means the product also protects against UVA. ## How does photodynamic therapy with aminolevulinic acid kill abnormal skin cells? Cells convert the prodrug to protoporphyrin IX; light of the right wavelength then makes it produce reactive oxygen species that destroy the cells. ## Why is salicylic acid used with care on large areas of a child's skin? Enough can be absorbed through the skin to cause salicylate poisoning, especially in young children and in kidney or liver disease. [Add to my cards (5)](https://durgical.app/estudiar/?add=/en/pharmacology/topics/dermatological-pharmacology-skin-vehicles-and-light/) [More Pharmacology questions →](https://durgical.com/en/practice/?f=pharmacology) ## Example drugs - [Methoxsalen](https://durgical.com/en/pharmacology/methoxsalen/) - [Clobetasol](https://durgical.com/en/pharmacology/clobetasol/) - [Gabapentin](https://durgical.com/en/pharmacology/gabapentin/) - [Naltrexone](https://durgical.com/en/pharmacology/naltrexone/) - [Crisaborole](https://durgical.com/en/pharmacology/crisaborole/) ## Connections ## Related classes 5 - [Corticosteroids (glucocorticoids)](https://durgical.com/en/pharmacology/classes/corticosteroids/) - [Dermatological retinoids](https://durgical.com/en/pharmacology/classes/dermatological-retinoids/) - [Other dermatological medicines](https://durgical.com/en/pharmacology/classes/other-dermatological-medicines/) - [H1 antihistamines](https://durgical.com/en/pharmacology/classes/h1-antihistamines/) - [Gabapentinoids](https://durgical.com/en/pharmacology/classes/gabapentinoids/) [← All of pharmacology](https://durgical.com/en/pharmacology/) Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given. Sources: [Sunscreen: How to Help Protect Your Skin from the Sun](https://www.fda.gov/drugs/understanding-over-counter-medicines/sunscreen-how-help-protect-your-skin-sun) (U.S. Food and Drug Administration, 2026); [Photodynamic Therapy to Treat Cancer](https://www.cancer.gov/about-cancer/treatment/types/photodynamic-therapy) (National Cancer Institute, 2026). ## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake Representative model, for educational purposes only. --- Source: https://durgical.com/en/pharmacology/topics/dermatological-pharmacology-skin-vehicles-and-light/ License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/pharmacology/topics/dermatological-pharmacology-skin-vehicles-and-light/