Topical Vehicles & Potency: Ointment, Cream, Gel — and Why the Base Is a Drug Decision

What a vehicle actually is

A topical drug is an active molecule plus the base that carries it — and the base is never inert. The vehicle is the formulation the active drug is dispersed in: the balance of oil and water, the emulsifiers, alcohols and preservatives around the molecule. It does three pharmacological jobs at once. First, it governs how readily the drug partitions out of the base and into the stratum corneum — the dead, waxy outer layer that is the rate-limiting barrier of the skin, as covered in the Skin barrier chapter. Second, it changes the skin itself: an oily base seals in water and hydrates the barrier, a drying base does the opposite. Third, it decides the cosmetic experience — greasy versus elegant, visible versus invisible — which quietly determines whether the patient keeps using it. Choosing a vehicle is therefore a pharmacokinetic decision dressed up as a texture preference.

The spectrum: from ointment to solution

Line the vehicles up by their oil-to-water ratio and everything else falls into place. Ointments are mostly oil (e.g. white soft paraffin) with little or no water. They are occlusive — they seal the surface, trap water, and hydrate the stratum corneum, and that hydration is itself what drives the strongest drug penetration. They are ideal for thick, dry, scaly, lichenified skin, but greasy and cosmetically poor. Creams are oil-and-water emulsions held together by an emulsifier; they spread easily, feel elegant, rub in without residue, and suit most body sites and weeping or intertriginous (flexural) areas. Lotions and solutions are largely water- or alcohol-based, thin and drying; they cover large or hairy areas (scalp) and wet, oozing lesions where you want evaporation, not occlusion. Gels are semisolid, often alcohol-based, transparent and non-greasy — excellent for the face, scalp and acne-prone skin, though the alcohol can sting fissured or eczematous skin. Foams and sprays are pressurised, quick-drying, patient-friendly vehicles that spread over hair-bearing skin with almost no residue — their whole reason to exist is adherence.

The same molecule, more potent in an ointment

This is the single most exam-relevant idea in the topic, and it surprises students every time: an identical corticosteroid molecule at the same concentration can sit in different potency classes depending only on its vehicle. Betamethasone or hydrocortisone in an occlusive ointment delivers more drug across the barrier than the same percentage in a cream or lotion, so the ointment behaves as if it were a stronger preparation — often effectively a rung higher on the potency ladder detailed in the Topical Corticosteroids chapter. The mechanism is pure percutaneous pharmacokinetics: occlusion plus hydration raises the fraction of drug that partitions into and diffuses through the stratum corneum. It is the topical equivalent of a bioavailability change — the same principle taught in the Principles of Pharmacology chapter, where formulation alters how much of a fixed dose actually reaches its site of action. The corollary is practical: switching a patient from cream to ointment can intensify therapy without ever changing the drug or its labelled strength — and, on thin facial or flexural skin, can also tip a "safe" steroid into causing atrophy.

What else changes how much drug gets in

The vehicle is one lever; percutaneous absorption has several others the prescriber controls. Occlusion — sealing the treated skin under a dressing, gloves, or the natural occlusion of a skin fold — can multiply penetration many-fold by hydrating the barrier and blocking evaporation; it is deliberately used to boost potent steroids on stubborn plaques, and inadvertently created by nappies or intertriginous areas. Hydration of the stratum corneum itself increases flux, which is why emollients applied first can enhance a later active. Surface area and regional variation matter enormously: absorption through thin eyelid, scrotal or flexural skin can be an order of magnitude greater than through thick palms or soles, so the same tube is high-risk on the face and barely effective on the heel. Skin integrity is the sleeper: inflamed, eroded, fissured or barrier-defective skin — precisely the skin being treated — has lost its rate-limiting layer, so systemic absorption climbs, and treating a large inflamed body-surface area in an infant is a genuine route to systemic steroid effects. Finally, chemical penetration enhancers such as propylene glycol, and keratolytics like salicylic acid or urea that dissolve the horny layer, are sometimes formulated in on purpose to open the barrier.

The fingertip unit: dosing something you smear

A topical can be under-dosed (no effect, blamed on the drug) or over-dosed (atrophy, systemic absorption) just as easily as a tablet — but "apply a thin layer" gives the patient no real quantity. The fingertip unit (FTU) fixes that: it is the amount of ointment or cream squeezed from a standard 5 mm nozzle along an adult's fingertip, from the tip to the first crease — roughly half a gram. One FTU covers an area about the size of two flat adult hands. From there, whole regions are counted in FTUs: a hand or foot takes about one, an arm about three, a leg about six, the trunk (front or back) about seven each. The FTU turns a vague smear into a reproducible dose, lets you estimate how long a tube should last (and thus spot both under-use and dangerous over-use), and is the practical bridge between potency theory and what the patient actually applies at home.

Adherence: the vehicle patients will actually use

The most potent, best-matched preparation does nothing sitting unused in the bathroom cupboard. Cosmetic acceptability is not vanity — it is a hard determinant of outcome, and topical adherence is notoriously poor. A greasy ointment that stains clothing, ruins make-up, or shines on the face at work will be quietly abandoned, whatever its pharmacological superiority; a non-greasy gel, foam or lotion that the patient is willing to apply twice daily beats a theoretically stronger ointment used twice a week. Simplicity helps too: once-daily regimens and combined preparations are used more reliably than complex multi-step routines. This is why modern formulation science invests so heavily in foams, sprays and elegant creams — matching the vehicle to the patient's life is often the difference between a treatment that works and one that only should. The emollient (moisturiser) base deserves the same respect: bland emollients in ointment, cream or lotion form are first-line therapy in eczema and ichthyosis in their own right, restoring the barrier discussed in the Skin barrier chapter, and their success too rests on choosing a texture the patient will apply generously and often.