Topical drugs (drugs applied directly to the skin) are a mainstay of treating skin disorders. Systemic drugs are taken by mouth or given by injection and are distributed throughout the body. Rarely, when a high concentration of a drug is needed at the affected area, a doctor injects the drug just under the skin (intradermal injection).
The active ingredient, or drug, in a topical preparation is mixed with an inactive ingredient (vehicle). The vehicle determines the consistency of the product (for example, thick and greasy or light and watery) and whether the active ingredient remains on the surface or penetrates the skin. Depending on the vehicle used, the same drug can be placed in an ointment, cream, lotion, solution, gel, oil, foam, or powder. In addition, many preparations are available in different strengths (concentrations).
Ointments (such as petroleum jelly) are oily and contain very little water. They are messy, greasy, and difficult to wash off. Ointments are most appropriate when the skin needs lubrication or moisture. Ointments are usually better than creams at delivering active ingredients into the skin. A given concentration of a drug is more potent in an ointment than in a cream. Ointments are less irritating than creams and much less irritating than gels, lotions, and solutions for open wounds such as erosions or ulcers.
Creams, the most commonly used preparations, are emulsions of oil in water, meaning they are primarily water with an oil component. (An ointment is the opposite, some water mixed mostly with oil.) Creams are easy to apply and appear to vanish when rubbed into the skin. They are relatively non-irritating.
Lotions are similar to creams but contain more water. They are actually suspensions of finely dispersed, powdered material in a base of water or oil and water. They are less effective than ointments, creams, and gels at delivering drugs and are considered of lower potency for a given drug concentration. Lotions have a number of beneficial effects. They are easy to apply to hairy skin, and they are particularly useful for cooling or drying inflamed or oozing lesions, such as those caused by contact dermatitis, athlete's foot (tinea pedis), and jock itch (tinea cruris).
Baths and soaks are used when treatment must be applied to large areas of the body. This technique is most often used in the form of sitz baths for over-the-counter (OTC) treatments of mild skin problems such as hemorrhoids. Baths are not often used to apply potent prescription drugs because of difficulties controlling the amount of drug delivered.
Solutions are liquids in which a drug is dissolved. The most commonly used liquids are alcohol, propylene glycol, polyethylene glycol, and plain water. Solutions are convenient to apply but tend to dry rather than moisturize the skin. However, this drying effect is useful for wet, oozing (weeping) skin disorders. Depending on the vehicle used, solutions can be irritating to the skin, particularly when those containing alcohol and propylene glycol are applied to open wounds.
Powders are dried forms of substances that are used to protect areas where skin rubs against skin—for instance, between the toes or buttocks, in the armpits or groin, or under the breasts. Powders are used on skin that has been softened and damaged by moisture (macerated). They may be mixed with active drugs such as antifungals.
Gels are water- or alcohol-based substances thickened without oil or fat. The skin does not absorb gels as well as it absorbs preparations containing oil or fat. Gels tend to be quite irritating on open wounds and diseased skin.
Types of Topical Drugs
Topical drugs can be divided into several overlapping categories: cleansing agents, protective agents, moisturizing agents, drying agents, anti-itch agents, anti-inflammatory agents, anti-infective agents, and keratolytics.
The principal cleansing agents are soaps, detergents, and solvents (a liquid substance capable of dissolving other substances). Soap is the most popular cleanser, but detergents are used as well. Certain soaps dry the skin; others have a creamy base that is less drying.
Because baby shampoos are excellent cleansing agents and are usually gentle to the skin, they are good for cleansing wounds, cuts, and abrasions. Also, people who have psoriasis, eczema, and other scaling diseases can use baby shampoos to wash away dead scaly skin. Oozing lesions, however, should generally be cleansed only with water and gentle soaps because detergents and harsher soaps can irritate the area.
Many chemicals are added to cleansing agents. For example, some soaps have antibacterial substances added to them. Antibacterial soap does not improve hygiene or prevent disease, and routine use may disrupt the normal balance of bacteria on the skin. Antidandruff shampoos and lotions may contain zinc dipyrithione, selenium sulfide, or tar extracts to help treat flaking skin, eczema, and psoriasis of the scalp.
Water is the main solvent for cleansing. Other solvents include petroleum jelly, which can cleanse the skin of material that cannot be dissolved with soap and water, such as tar. Small amounts of alcohol can safely be used to cleanse the skin before injections or blood drawing. Alcohol gels are useful for routine hand hygiene when handwashing is not possible. Other solvents, such as acetone (nail polish remover), gasoline, and paint thinner, are rarely used for skin cleansing. These solvents dissolve the skin's natural oils, causing significant drying and irritation. They may also be absorbed through the skin, resulting in poisoning.
Many different kinds of preparations help protect the skin. Oils and ointments supply an oil-based barrier that can help protect scraped or irritated skin and retain moisture. Powders may protect skin that rubs against skin or clothing. Synthetic hydrocolloid dressings protect pressure sores (bedsores, decubitus ulcers) and other areas of raw skin. Sunscreens and sunblocks reflect, absorb, or filter out harmful ultraviolet light.
Moisturizers (emollients) restore and help maintain water and oils in the skin. The best time to apply a moisturizer is when the skin is already moistened—immediately after a bath or shower, for instance. Moisturizers typically contain glycerin, mineral oil, or petrolatum and are available as lotions, creams, ointments, and bath oils. Some stronger moisturizers contain compounds such as urea, lactic acid, and glycolic acid.
Excessive moisture in areas where skin rubs against skin can cause irritation and skin breakdown (maceration), particularly in body folds where the environment tends to be warmer and moister. The areas most commonly affected are between the toes or buttocks, in the armpits or groin, and under the breasts and abdominal skin folds. These warm moist areas also provide fertile breeding grounds for infections, especially with fungi and bacteria.
Talcum powder is the most commonly used drying agent. Talc absorbs moisture from the skin surface. Most of the many talc preparations vary only in their scents and packaging. Cornstarch is another good drying agent. Talc is usually preferred, except for babies, because babies can accidentally inhale the powder, and cornstarch is less dangerous to breathe than talc.
Solutions containing aluminum salts are drying agents commonly found in OTC antiperspirants. Prescription doses of aluminum salts are available to treat excessive sweating.
Astringents are liquid drying agents that narrow blood vessels. The most commonly used astringent solutions contain aluminum acetate (Burow's solution or Domeboro's solution). Usually applied with dressings or as soaks, astringents are used to treat infectious eczema, oozing skin lesions, and pressure sores. Witch hazel is also a popular OTC astringent.
Skin disease is often accompanied by itching. Itching and mild pain can sometimes be controlled with soothing agents such as chamomile, eucalyptus, camphor, menthol, zinc oxide, talc, glycerin, and calamine. These are available as OTC preparations.
Antihistamines, which block certain types of allergic reactions, are sometimes included in topical preparations to relieve the itching associated with allergic reactions. Doxepin is an effective topical antihistamine for many conditions. However, the antihistamine diphenhydramine (common in many nonprescription topical preparations) can trigger an allergic reaction when applied to the skin and is usually not recommended. Taking antihistamines by mouth does not seem to produce this type of reaction, so oral rather than topical antihistamines are preferred to relieve itching.
Corticosteroids are the main topical drugs used to relieve inflammation (swelling, itching, and redness) of the skin. Corticosteroids are most effective for rashes caused by allergic or inflammatory reactions to things such as poison ivy, metals, cloth, drugs, eczema, and many others. Because they lower resistance to bacterial and fungal infections and inhibit wound healing, corticosteroids usually should not be used on infected areas or wounds. For acne-like disorders, topical corticosteroids tend to not work very well and sometimes instead induce an acne-like eruption. Corticosteroids are sometimes mixed with antifungal drugs to help reduce redness and itching while simultaneously eradicating the fungus.
Topical corticosteroids are sold as lotions, creams, ointments, solutions, foams, oils, and gels. Creams are most effective if rubbed in gently until they vanish. In general, ointments are the most potent. The type and concentration of corticosteroid in the preparation determines the overall strength. Hydrocortisone is available in concentrations of up to 1% without a prescription; concentrations of 0.5% or less offer little benefit. Stronger corticosteroid preparations require a prescription. Doctors usually prescribe potent corticosteroids first, then less potent corticosteroids as the disorder improves. Generally, topical corticosteroids are applied 2 to 3 times a day in a thin layer, but high-potency formulations may be applied only once a day.
Corticosteroids should be used with caution on areas where the skin is thin, such as the face, and on areas of natural occlusion, such as the armpits and groin. Doctors usually use low-potency corticosteroids on these sensitive areas for no more than a few days to a week (see see Sidebar 3: Corticosteroids: Uses and Side Effects). Prolonged use (more than 1 month) in any area can cause skin breakdown, stretch marks, acne-like eruptions, and sometimes an allergic skin reaction (contact dermatitis) to the corticosteroid itself. Perioral dermatitis (a red, bumpy rash around the mouth, chin, and sometimes the eyes) occurs more commonly with mid-potency or high-potency formulations used on the face and less commonly with mild formulations. High-potency formulations may inhibit adrenal gland functions when used in children, when used over large areas of skin, or when used for long periods of time, especially if used under occlusive dressings.
When a stronger dose of topical corticosteroid is needed for one spot or a small area that does not respond to treatment, a doctor may inject the corticosteroid just under the skin or occasionally apply plastic tape infused with the corticosteroid flurandrenolide. Another way to deliver a strong dose is to apply a thin plastic film, such as household plastic wrap, over the topical corticosteroid (occlusive dressing). The plastic film increases the drug's absorption and effectiveness and is usually left on overnight. Such dressings are usually reserved for disorders such as severe psoriasis and eczema. Risks of using corticosteroids under an occlusive dressing include development of prickly heat (miliaria), skin thinning (atrophy), stretch marks (striae), dilated red blood vessels on the surface of the skin (telangiectasias), and bacterial or fungal infections.
Several allegedly anti-inflammatory herbal products are commonly used in commercial products, although their effectiveness has not been well established. Herbal and "natural" products are often not standardized and commonly cause allergic and irritant reactions of the skin. Among the most popular are chamomile and calendula.
Tar preparations, which are byproducts of coal manufacturing, slow skin cell division and are useful in treating disorders that cause excess skin production (scaling) such as psoriasis. Side effects include irritation, inflammation of follicles (folliculitis), staining of clothes and furniture, and sensitivity to sunlight (photosensitization). They should not be used on infected skin.
Viruses, bacteria, fungi, and parasites can all infect the skin. By far, the best way to prevent such infections is by carefully washing the skin with soap and water. Stronger disinfecting agents are commonly used by nurses and doctors to disinfect their hands to prevent spreading infections to patients. Antibacterial preparations or "preps" are used on the skin before surgery to lower the number of bacteria on the skin and thereby prevent postoperative infections. Once a skin infection has occurred, it may be treated with topical or systemic drugs depending on the severity and type of infection diagnosed or suspected. Topical anti-infective agents include antibiotics, antifungals, and insecticides.
Topical antibiotics have few uses. Clindamycin and erythromycin are sometimes used as primary or additional treatment for acne. Mupirocin can be used to treat impetigo (a staphylococcal infection of the skin). Nonprescription antibiotics such as bacitracin and polymyxin are often used in postoperative care of a skin biopsy site and to prevent infection in scrapes, minor burns, and abrasions. Although considered generally quite safe, topical antibiotics do have some side effects. For example, neomycin (a common ingredient in nonprescription antibiotic ointments) frequently causes an allergic reaction.
Topical antifungals work quite well for treating a wide variety of fungal infections of the skin (such as ringworm and athlete's foot). However, these topical drugs work poorly for treating fungal infections of the nails. Typically, nail infections are treated with oral antifungals (usually terbinafine), but relapse is very common even when oral drugs are taken.
Insecticides (such as permethrin and malathion) are used to treat lice infestations and scabies.
Non-antibiotic topical antiseptics include iodine solutions (such as povidone iodine and clioquinol), gentian violet, silver preparations (such as silver nitrate and silver sulfadiazine), and zinc pyrithione. Iodine is used to prepare the skin for surgery. Gentian violet is used when an inexpensive and chemically and physically stable antiseptic, antimicrobial, or both is needed. Silver preparations (such as silver sulfadiazine) are effective in treating burns and ulcers and have strong antimicrobial properties; several wound dressings are infused with silver. Zinc pyrithione is an antifungal and a common ingredient in shampoos that treat dandruff caused by an overgrowth of a common skin fungus. Healing wounds should usually not be treated with topical antiseptics other than silver because they are irritating and tend to kill fragile regrowth (granulation tissue).
Keratolytics are agents that soften skin cells and ease the flaking and peeling process. Examples include salicylic acid and urea.
Salicylic acid in varying concentrations is used to treat psoriasis, seborrheic dermatitis, acne, and warts. Side effects are common and include burning, irritation, and systemic toxicity if large areas of skin are covered. Salicylic acid should rarely be used in children and infants.
Urea can be used to moisturize, sooth itching, and reduce scaling. It is commonly used to treat excessive skin build up on the soles of the feet (plantar keratodermas and calluses), keratosis pilaris (dry bumps on thighs and back of arms in people with allergies), and severe dry skin (ichthyosis). Side effects are irritation and burning. Urea should not be applied to large areas of skin.
Dressings protect open wounds, facilitate healing, increase drug absorption, and protect clothing. Dressings are nonocclusive (air can reach the wound) or occlusive (wounds are covered and sealed from contact with air).
The most common nonocclusive dressings are gauze dressings. They maximally allow air to reach the wound and allow the wound to dry. Nonocclusive dressings wetted with solution, usually saline, are used to help cleanse and remove (debride) thickened, crusted, or dead tissue. The dressings are applied wet and removed after the solution has evaporated (wet-to-dry dressings). The dried materials stick to the dressing.
Occlusive dressings increase the absorption, potency, and effectiveness (and side effects) of topical drugs. Transparent impermeable films such as polyethylene (plastic household wrap) or flexible, transparent, semi-permeable dressings are the most common types of occlusive dressings. Hydrocolloid dressings are used to speed the healing of skin ulcers. Zinc oxide gelatin (Unna's paste boot) is an effective occlusive dressing for skin inflammation and ulcers of the lower legs (which can occur in stasis dermatitis). Occlusive dressings are sometimes recommended for treating severe psoriasis, atopic dermatitis, skin lesions of lupus erythematosus, and chronic hand dermatitis, among other conditions.
Other occlusive dressings are used to protect and help heal burns. Doctors have recently discovered that other types of open wounds also heal faster and more completely when kept moist and under an occlusive dressing. These dressings help maintain a proper level of moisture and provide a framework on which new skin can regrow. Such dressings include sophisticated commercial products as well as plain petroleum jelly or an antibiotic ointment under a bandage.
Last full review/revision October 2006 by Robert J. MacNeal, MD