The vehicle for many topical medicaments is white petrolatum. Petrolatum is derived from petroleum distillation and is made up of long-chain aliphatic hydrocarbons. There are two forms, white and yellow petrolatum, with the difference being the amount of refining/impurities present. White petrolatum is the most refined and most commonly is used as a skin protectant and moisturizing agent, as well as the base for creams and lotions. In its pure form, white petrolatum does not require preservatives and is not a cause of allergic contact dermatitis (ACD). Yellow petrolatum is a rare cause of ACD. The different vehicle types are discussed below and are also summarized in Table 2-1.

White petrolatum is used most often as a base for ointments. Many topical medicaments, in addition to the white petrolatum base, add humectants such as propylene glycol (PG) or glycerin. Other lipophilic bases are sometimes used for topical medicaments. Beeswax, lanolin, and natural oils (jojoba, sweet almond, peanut oil, and others) are infrequently used as a base for delivery of medicaments. PG is the base for water-soluble products and is also added to lipophilic ointments and creams to increase potency and penetration of the epidermis, as well to act as a humectant. Corticosteroid preparations often add PG. Lipophilic ointments are an effective barrier for water loss for 4 to 6 hours after application. They are occlusive, provide excellent hydration, and increase penetration of the epidermis if a medication such as a corticosteroid is part of the ointment system.

Creams, at their simplest, are white petrolatum or other oil with added water. This addition makes the cream less greasy and more easily rubbed/absorbed into the skin, making creams much more accepted by patients compared to ointments. Because of the added water content, addition of a preservative agent to prevent spoilage is necessary. These agents prevent microbial overgrowth, but also may cause allergic sensitization. Common additives are summarized in Table 2-2. Creams are most commonly used in clinical practice due to their

great patient acceptance. They are less hydrating compared to ointments and are poorly tolerated on hair-bearing skin such as the scalp or thicker trunk hair due to matting of the hair.

Lotions are thick liquids, composed of water or alcohol bases with added ingredients. A lotion may simply be an oil-in-water emulsion, basically a cream with further added water for even easier application. A “shake lotion” is a powder-in-water suspension, the most common being Calamine lotion. This is composed of zinc oxide, talc, glycerin, water, ferric oxide, bentonite magma, and calcium hydroxide. Shake lotions are excellent for use in areas to which application is challenging, such as hair-bearing skin or scalp. Shake lotions traditionally have been useful in exudative, inflammatory processes to which creams are challenging to apply. In addition, lotions are useful in conditions where drying of the affected skin is desirable, such as in an exudative intertrigo. Lotions with an alcohol base are helpful for pruritic skin as they are cooling and astringent. A common example of such a lotion is the commercially available Sarna lotion.

Gels are vehicles that are commonly used on sebaceous areas of the body such as the face and chest or for hair-bearing areas such as the scalp. The main component of a gel is water, acetone, alcohol, or propylene glycol with organic polymers suspended with substances such as agar, gelatin, hydroyxpropylcellulose, carbomer methylcellulose, pectin, and/or polyethylene glycol. Gels are semi-solid on application but then melt due to body heat and are absorbed into the skin without leaving a film. This base enhances penetration and absorption through the epidermis for the medicament contained in the gel (often a corticosteroid or retinoid preparation). In some individuals, gels may cause mild xerosis of the skin.

Powders are finely particulate, hygroscopic substances that are used to dry moist skin areas and minimize friction between opposing surfaces. A common location for the use of a powder would be in intertriginous areas such as the inguinal crease, axillae, or under skin folds in larger individuals. Common bases include zinc oxide, zinc or magnesium stearate, talc, cornstarch, or precipitated calcium carbonate. Some powders contain added medication, often an antibacterial or antimycotic agent.

Pastes are powder mixed with an ointment. The most common dermatologic paste is zinc oxide compounded in white petrolatum. Pastes are useful for skin protection such as in a diaper or other areas where irritant contact dermatitis can occur, or as a sunscreen. Zinc oxide paste is opaque when applied and is a broad-spectrum ultraviolet (UV) blocker. Micronized zinc oxide is more cosmetically acceptable as visible light passes through, but UVA and UVB are blocked.

Application techniques and choice of base depend on the type of condition being treated and body site on which the medication will be applied (Table 2-3). The most basic distinction is whether the area to be treated is exudative or xerotic. For dry skin or conditions where moisturization is desired, the best base to choose is an ointment. Use after bathing/showering helps to seal moisture in the skin and the occlusive effect of the ointment helps retain the moisture. If a medicated ointment is used, absorption through the epidermis is greater than for a cream of the same potency. Creams are useful as less sticky/messy variants of ointments. Many patients will not use an ointment or will use it too sparingly due to its long-lasting stickiness and potential to stain clothing. Due to the increased water content, creams will absorb readily into the skin. For exudative lesions/conditions, use of a lotion or gel is useful, as they have drying properties. Another factor to keep in mind is the amount of hair in the area of application.
For patients with significant amounts of body hair or for scalp/pubic application, lotions, solutions, or foams are much more acceptable. Creams and ointments will mat the hair and will be challenging to apply.