Bases are classifi ed based on their composition and physical characteristics. The U.S.
Pharmacopeia (USP) classifi es ointment bases as hydrocarbon bases (oleaginous bases), absorption bases, water - removable bases, and water - soluble bases (water - miscible bases) [1] .
Hydrocarbon bases are made of oleaginous materials. They provide emollient
and protective properties and remain in the skin for prolonged periods. It is diffi cult
to incorporate aqueous phases into hydrocarbon bases. However, powders can be incorporated into these bases with the aid of liquid petrolatum. Removal of hydrocarbon bases from the skin is diffi cult due to their oily nature. Petrolatum USP, white petrolatum USP, yellow ointment USP, and white ointment USP are examples of hydrocarbon bases.
Absorption bases contain small amounts of water. They provide relatively less
emollient properties than hydrocarbon bases. Similar to hydrocarbon bases, absorption bases are also diffi cult to remove from the skin due to their hydrophobic nature.
Hydrophilic petrolatum USP and lanolin USP are examples of absorption bases.
Water - removable bases are basically oil - in - water emulsions. Unlike hydrocarbon and absorption bases, a large proportion of aqueous phase can be incorporated into water - removable bases with the aid of suitable emulsifying agents. It is easy to remove these bases from the skin due to their hydrophilic nature. Hydrophilic ointment USP is an example of a water - removable ointment base.
Water - soluble bases do not contain any oily or oleaginous phase. Solids can be easily incorporated into these bases. They may be completely removed from the skin due to their water solubility. Polyethylene glycol (PEG) ointment National
Formulary (NF) is an example of a water - soluble base.
Selection of an appropriate base for an ointment or cream formulation depends on the type of activity desired (e.g., topical or percutaneous absorption), compatibility with other components, physicochemical and microbial stability of the product, ease of manufacture, pourability and spreadability of the formulation, duration of contact, chances of hypersensitivity reactions, and ease of washing from the site of application. In addition, bases that are used in ophthalmic preparations should be nonirritating and should soften at body temperatures. White petrolatum and liquid petrolatum are generally used in ophthalmic preparations. Table 1 summarizes
compendial status, synonym, and specifi cations of some of the bases used in ointments and creams.
The following sections describe the source, physicochemical properties, formulation considerations, stability, incompatibility, storage, and hypersensitivity reactions of some of these bases.
Lanolin Lanolin is a refi ned, decolorized, and deodorized material obtained from sheep wool. It is available as a pale yellow, waxy material with a characteristic odor.
It is extensively used in the preparation of hydrophobic ointments and water - in – oil creams. As lanolin is prone to oxidation, antioxidants such as butylated hydroxytoluene are generally included. Although lanolin is insoluble in water, it is miscible with water up to 1 : 2 ratio. This property favors in preparing physically stable creams.
Addition of soft paraffi n or vegetable oil improves the emollient property of lanolin preparations. Exposure of lanolin to higher temperature usually leads to discoloration and rancidlike odor, and hence prolonged heating is avoided during the
preparation and preservation of lanolin - containing preparations. Gamma sterilization or fi ltration sterilization is usually employed for sterilizing ophthalmic ointments containing lanolin. Lanolin and some of its derivatives are reported to cause hypersensitivity reactions and therefore are avoided in patients with known hypersensitivity.
One of the reasons for hypersensitivity reactions is free fatty alcohols.
Modifi ed lanolins containing reduced levels of free fatty alcohols are commercially available [2, 3] .