Topical medications are widely used to treat dermatological conditions, but their effectiveness depends on more than simply selecting the correct drug. The vehicle used to deliver the medication, the location of application, skin thickness, and the condition of the skin can all significantly influence how much medication is absorbed.
Understanding these factors is important when prescribing topical therapies because the same medication may produce substantially different levels of absorption when applied to different areas of the body or formulated in different vehicles.
The skin does not have uniform permeability throughout the body. Some anatomical regions allow topical medications to penetrate much more readily than others.
Areas such as the face, axillae, and genital region are relatively permeable and therefore tend to absorb topical medications more readily. In contrast, areas with a thicker outer skin barrier, such as the palms of the hands and soles of the feet, allow considerably less medication to penetrate.
This difference is largely related to the thickness of the stratum corneum, the outermost layer of the epidermis and an important barrier to drug penetration.
Clinical Pearl: Topical medication absorption is generally inversely related to the thickness of the stratum corneum. Thinner skin tends to permit greater absorption, while thicker skin provides a stronger barrier to penetration.
The effect of anatomical location can be substantial. For example, hydrocortisone absorption from the forearm may be less than one-third of the amount absorbed from the forehead. This demonstrates why the location being treated should be considered when selecting the medication, concentration, vehicle, frequency, and duration of topical therapy.
This principle is particularly important with medications such as topical corticosteroids, where excessive absorption may increase the risk of local adverse effects and, with sufficiently extensive or prolonged exposure, systemic effects.
A topical medication consists not only of its active pharmaceutical ingredient but also of a vehicle that delivers the drug to the skin. Common vehicles include lotions, creams, and ointments. The choice of vehicle influences both drug absorption and the practical characteristics of treatment.
As a general principle, less viscous vehicles tend to result in less medication absorption. Therefore, the same medication may be absorbed differently depending on whether it is formulated as a lotion, cream, or ointment.
| Vehicle | Relative Absorption | Clinical Characteristics |
|---|---|---|
| Lotion | Lower | Less viscous; generally results in less medication absorption |
| Cream | Intermediate | More substantial vehicle than lotion; commonly used for many dermatologic conditions |
| Ointment | Higher | Generally allows greater drug absorption and provides additional lubrication to the skin |
Among commonly used topical vehicles, ointments generally allow for greater drug absorption. Their more occlusive nature helps retain moisture within the skin and facilitates penetration of the therapeutic agent.
Ointments also provide lubrication to the treated area. This can be particularly beneficial when a dermatologic disorder is associated with xerosis, or excessively dry skin, because the vehicle itself helps reduce moisture loss and improve skin hydration.
The moisturizing and lubricating properties of creams and especially ointments may provide additional therapeutic benefits beyond delivery of the active medication. This is often desirable when treating xerosis or lichenification.
Lichenification refers to thickened, leathery skin with accentuated skin markings that commonly develops following chronic rubbing or scratching. In these situations, selecting an appropriate vehicle can help address both the underlying inflammatory process and the impaired skin barrier.
Key Point: Prescribing a topical medication involves more than choosing the active drug. Clinicians should consider where the medication will be applied, the permeability and thickness of the skin at that site, and the vehicle carrying the medication. These factors can substantially alter drug absorption and ultimately affect both treatment efficacy and safety.
Robertson D, Maibach H. Dermatologic pharmacology. In: Katzung B, ed. Katzung’s Basic and Clinical Pharmacology. 14th ed. New York, NY: McGraw-Hill Medical; 2018:1068–1086.
