ointment base
The ointment base is the “body” of an ointment — the bulk material that carries the drug, like the bread in a sandwich into which the filling is folded. By itself it has no active drug, but the base decides how the product feels, how long it stays on, and how well the drug gets into the skin.
It is the inert vehicle in which the active ingredient is dissolved or dispersed. Bases fall into broad families: hydrocarbon (greasy, occlusive) bases like petrolatum that seal in moisture; absorption bases that contain wool fat (lanolin) and can soak up some water; water-removable (emulsion) bases that wash off; and water-soluble bases such as polyethylene glycol that contain no oil at all. Each family gives a different feel, occlusiveness and drug-release profile.
Choosing the base is one of the most consequential decisions in a topical formulation, because it governs occlusion, skin hydration, how readily the drug partitions into the skin, and patient acceptability. A drug that releases well from a greasy base may release poorly from a water-soluble one, so the base is selected to match both the drug and the clinical purpose, not treated as a neutral filler.
White soft paraffin is a hydrocarbon ointment base: highly occlusive and inert, it suits dry chronic conditions, whereas a polyethylene-glycol base is chosen when a non-greasy, water-soluble, washable vehicle is wanted.
The base, not the drug, decides whether an ointment is greasy and occlusive or washable.
The base is not a passive filler: it controls occlusion, skin hydration and how the drug partitions out of the vehicle, so the same drug can behave very differently in different bases.