Topical medications are absorbed by passing through the outermost layer of skin, a barrier called the stratum corneum that is only 10 to 20 micrometers thick. Despite being thinner than a sheet of paper, this layer is the single biggest obstacle a topical drug has to cross. How well it crosses depends on the drug’s molecular size, the formulation it’s mixed into, and the condition of your skin at the time of application.
The Skin Barrier That Controls Absorption
The stratum corneum is made of dead, flattened skin cells packed tightly together and surrounded by a matrix of lipids (natural fats). Think of it like a brick wall: the cells are the bricks, and the lipids are the mortar. Any drug applied to the surface has to find a way through this structure before it can reach the living cells underneath, and eventually the tiny blood vessels in the deeper layer of skin called the dermis.
There are three routes a drug molecule can take through this barrier. The first is the intercellular route, where the drug winds through the lipid “mortar” between cells. This is the most common pathway for most topical drugs. The second is the transcellular route, where molecules pass directly through the skin cells themselves. The third is the appendageal route, where drugs slip through hair follicles and the glands attached to them. Hair follicles and sweat glands are essentially tunnels that bypass the stratum corneum and connect directly to deeper skin tissue. This makes them especially important for larger molecules that can’t easily squeeze between or through cells.
Why Molecular Size Matters
There is a well-established threshold in dermatology known as the 500 Dalton rule. Molecules weighing less than 500 Daltons (a unit of molecular mass) can generally pass through the stratum corneum. Molecules above that cutoff essentially cannot. This isn’t just a theoretical guideline: virtually all topical drugs used in dermatology, and every drug in transdermal patch systems, falls below 500 Daltons. Common contact allergens are also all under this threshold, which is why larger proteins rarely cause skin allergies on contact.
Beyond size, a drug’s chemistry matters too. Molecules need to be somewhat fat-soluble to dissolve into the lipid matrix of the stratum corneum, but also somewhat water-soluble to eventually move into the watery living tissue below. Drugs that are extremely oily or extremely water-loving tend to get stuck at one stage or the other.
How the Formulation Changes Absorption
The vehicle, meaning the cream, ointment, gel, or lotion that carries the drug, plays a surprisingly large role in how much medication actually gets into your skin. The key factor is how well the vehicle traps moisture against the skin surface, a property called occlusivity. When skin stays hydrated, the stratum corneum softens and swells slightly, creating wider pathways for drug molecules to pass through.
Ointments are the most occlusive vehicle. They contain less than 20% water and more than 50% hydrocarbons or waxes, forming a greasy layer that seals moisture in. This makes them the most effective at driving drug penetration, particularly on thick, dry, or scaly skin. Creams contain more water (over 20%) and are less occlusive, so they deliver less drug into the skin compared to an ointment with the same active ingredient at the same concentration. They’re more comfortable to wear, though, which is why they’re prescribed more often for everyday use.
Gels liquefy on contact with warm skin and dry into a thin, greaseless film. They provide almost no occlusion or hydration, making them among the least potent vehicles for drug delivery. Lotions are even less occlusive, since they’re more than 50% water. Foams and sprays also provide minimal occlusion. The tradeoff is always comfort versus potency: ointments are the most effective but the greasiest, while gels, lotions, and sprays feel better on the skin but deliver less drug.
Chemical Penetration Enhancers
Many topical formulations include inactive ingredients specifically designed to help the drug cross the skin barrier more easily. Ethanol is the most widely used penetration enhancer. It works in two ways: it increases the concentration of the drug at the skin surface, and it disrupts the organized lipid structure in the stratum corneum, temporarily making the barrier more permeable.
Propylene glycol, another common ingredient, works differently. It improves how well the drug partitions (distributes) between the formulation and the skin, and it interacts with the water-containing portions of the lipid layers to change their structure. It also reduces how much drug gets bound up by keratin, the tough protein that makes up much of the stratum corneum, leaving more free drug available to move deeper. You’ll find one or both of these enhancers listed in the inactive ingredients of most topical prescription medications.
Where on the Body You Apply It
Absorption varies significantly depending on the application site, because skin thickness and follicle density differ across the body. Areas with thin skin and high follicle density, like the face, scalp, and groin, absorb drugs much more readily than areas with thick skin, like the palms and soles. This is why your doctor may prescribe a lower-strength steroid for your face than for your elbows.
Across the trunk, absorption tends to be more uniform. Studies comparing drug absorption from the abdomen, buttocks, lower back, lateral chest, and upper arm have found no significant differences between these sites. The upper thigh absorbs roughly 15% less than the abdomen, a small but measurable drop. In practical terms, if you’re using a transdermal patch or a topical that needs consistent absorption, most trunk sites will work equally well.
When Topical Drugs Reach the Bloodstream
Not all topical medications are meant to stay in the skin. Depending on the goal, topical drugs fall into a few categories. Some are designed specifically to sit on the surface and not be absorbed at all, like sunscreens. Others target the skin tissue itself, as with antifungal creams or acne treatments. A third group needs to penetrate deeper to reach muscles or joints, like topical pain relievers. And a fourth group is designed to cross all the way through the skin into blood vessels and circulate throughout the body, the way nicotine patches or hormonal patches do.
For drugs that do reach the bloodstream, the appendageal route is especially important. Hair follicles and sweat glands extend deep into the dermis, where capillaries are plentiful. Large molecules that can’t easily cross the stratum corneum on their own can sometimes enter via these appendages and reach the blood directly. This is one reason why topical medications occasionally cause systemic side effects, particularly when applied to large surface areas, on damaged skin, or under occlusive dressings that boost absorption.
Newer Delivery Methods
Microneedles represent one of the most significant advances in topical drug delivery. These are tiny needles, often made of dissolving materials, that painlessly pierce the stratum corneum and release their drug payload directly into the epidermis or upper dermis. By physically bypassing the skin’s main barrier, they allow much larger molecules to enter the skin than passive absorption would permit.
Standard dissolving microneedles have a limitation, though: once the needle dissolves and releases the drug, the medication can only spread through passive diffusion, with no further control over how deep it travels. Newer experimental designs are addressing this. One approach uses gas-generating microorganisms embedded in the microneedle to actively push drugs deeper into tissue, increasing delivery depth by over 200% and reaching up to 1,000 micrometers below the skin surface. This kind of active transport could eventually allow topical treatments to reach targets that currently require injections.
Factors That Increase or Decrease Absorption
Several everyday factors influence how much of a topical drug your skin absorbs:
- Skin hydration: Moist skin is more permeable. Applying a medication right after a shower, or covering it with a bandage, increases absorption.
- Skin integrity: Broken, inflamed, or diseased skin has a compromised barrier, which lets more drug through. Burns, eczema flares, and open wounds all dramatically increase absorption.
- Skin thickness: Thin-skinned areas like eyelids and inner wrists absorb far more than thick-skinned areas like palms.
- Surface area: The larger the area you cover, the more total drug enters your body. This matters most for potent medications like topical steroids.
- Contact time: Longer contact means more absorption. Washing a medication off after a few minutes delivers far less drug than leaving it on for hours.
Understanding these variables helps explain why the same medication in the same strength can have very different effects depending on how and where you use it. A mild steroid ointment applied to a large patch of inflamed skin on your inner arm after a hot shower will absorb substantially more drug than the same ointment dabbed on intact skin on your lower leg.