Semi-solid and topical dosage forms

In dermatology the vehicle is an active part of the treatment. The physical rule is simple: oil and water do not mix, so whichever phase is continuous determines how the product behaves. In an oil-in-water cream the continuous phase is water, so the product is washable, spreads easily and loses water by evaporation, which cools and dries the surface. In a water-in-oil cream the continuous phase is oil, so the product is greasier, resists washing, and leaves an oily barrier that slows water loss from the outermost skin layer. The same logic extends to the hydrophobic drug question: a hydrophobic drug dissolves more readily in the oily phase and is released more readily from a W/O cream than from an O/W cream. The vocabulary that helps is lipophilic, meaning fat-loving, and aqueous, meaning water-based. The choice rule from the notes becomes predictable rather than memorised: dry chronic inflammation responds to lipophilic vehicles such as ointments and pastes, while moist acute inflammation responds to aqueous vehicles such as creams and lotions; if the skin is dry use an ointment, if it is wet use a cream. The beginner’s misconception is that all creams are simply weaker ointments. They are different systems, not different strengths, and a patient with a weeping lesion who is given an occlusive ointment may deteriorate even though the same active ingredient is present in both products.

Ointments and pastes both form an unbroken, relatively water-impermeable film, and that shared property is why they are called occlusive. Occlusion prevents the escape of moisture, keeps the drug in contact with the skin for a prolonged period, and makes these forms suitable for hyperkeratotic, scaling and eczematous lesions. The difference is the powder. A paste contains a high concentration of insoluble powdered substances, usually not less than 20%, and that powder changes everything physical about the product. It makes the paste stiffer and more viscous, more absorptive, and — because a large fraction of the mass is solid — unable to melt or soften at body temperature. It also makes the film opaque, which is why a paste can act as a sun block or screen, while an ointment film is not. Clinically this translates into a division of labour: use a paste when protective action is required and when serous discharge must be absorbed, and use an ointment when a greasy emollient or a therapeutic drug must be held against the skin. Both are suitable for dry skin surfaces and burns, and pastes are also useful in hairy areas, in lesions with minor exudate and in folded skin areas such as the groin, the creases of the neck and the armpit. The misconception to retire is that a paste is just a thick ointment. The powder load changes melting behaviour, absorption and light transmission, and it also makes the paste greasier and harder to wash off, so the two are not interchangeable.