Ointment bases are classified into the following categories:

1. Oleaginous (hydrocarbon) bases
2. Absorption bases
3. Water-miscible bases
4. Water-soluble bases

Oleaginous (Hydrocarbon) bases: These bases consist of oils and fats and, hence, are hydrophobic in nature, with the least water absorption capacity. Oleaginous bases provide emollient and protective properties by restricting the loss of moisture from the skin and remaining on the skin for prolonged periods. They are occlusive in nature. Removal of hydrocarbon bases from the skin is difficult. They undergo no action with the skin and are economical. The following are the different types of oleaginous bases:

1. Petrolatum (Soft paraffin): It is a purified mixture of semisolid hydrocarbons obtained from petroleum or heavy lubricating oil. It is available in the form of white soft paraffin and yellow soft paraffin.
2. Hard paraffin (Paraffin): It is a chemical mixture of solid hydrocarbons obtained from petroleum. It is a colorless or white, odorless, translucent and waxlike substance, which solidifies between 50°C and 57°C. It can be used to stiffen ointment bases.
3. Liquid paraffin (Liquid petrolatum or white mineral oil): It comprises liquid hydrocarbons obtained from petroleum. It is a transparent, colorless, odorless and viscous liquid. Practically, it is used along with hard paraffin and soft paraffin to get desired consistency of ointment for easy extrudability from the collapsible tubes and spreadability on the affected part.

The advantages of hydrocarbons bases are as follows:

1. They maintain the skin soft by remaining on the surface as an occlusive layer and preventing the loss of moisture.
2. They ensure prolonged contact between skin and medicament due to their stickiness.
3. They are almost physicochemically inert, with little tendency of rancidity.
4. They can withstand heat sterilization. Hence, they can be employed in the formulation of sterile ophthalmic ointments.
5. They are easily available and very cheap.

The following are the disadvantages of hydrocarbon bases:

1. Prolonged use may result in water logging with maceration of the skin.
2. They retain body heat, thereby producing an uncomfortable feeling of warmth.
3. They are immiscible with water. Both application and removal after treatment are difficult.
4. They are sticky, making the application unpleasant, and might cause staining of clothes.
5. They are poor in absorbing exudate from moist lesions as their water absorption capacity is very low.

Absorption (Emulsifiable) ointment bases: The term “absorption base” signifies the water-absorbing or emulsifying property of these bases and does not describe their action on the skin. They contain relatively less emollient properties than hydrocarbon bases. They are hydrophilic in nature and have a tendency to absorb large quantities of water and aqueous solutions. Such preparations normally do not contain water as a component of their basic formula, but if water is incorporated, it results in a water-in-oil (w/o) emulsion.

1. Wool fat (Anhydrous lanolin): This is obtained from the wool of sheep as a purified anhydrous fat. It is practically insoluble in water but can absorb water up to 50% of its own weight. It is used along with other bases in the preparation of a number of ointments. For example, simple ointment I.P contains 5% and eye ointment base B.P contains 10% of wool fat.
2. Hydrous wool fat (Lanolin): This is a mixture of 70% w/w of wool fat and 30% w/w of purified water. It is a w/o type of emulsion. Hence, aqueous liquids can be emulsified with it. Examples are hydrous wool fat ointment B.P.C and calamine coal tar ointment.
3. Wool alcohol: This is an emulsifying fraction of wool fat. Wool alcohol is obtained by treating wool fat with alkali, with the separation of the fraction containing cholesterol and other alcohols. Normally, it contains not less than 30% of cholesterol. It is pharmaceutically employed as an emulsifying agent and to improve texture, stability and emollient properties in the preparation of w/o emulsions and absorb water in ointment bases. For example, wool alcohol ointment B.P contains 6% wool alcohol.
4. Beeswax: This is obtained from the honey comb of bees as a purified wax, which is available as yellow beeswax and white beeswax. It is used in ointment preparations as a stiffening agent. For example, paraffin ointment B.P.C contains beeswax.
5. Cholesterol: Animals are the main source for cholesterol as it is extensively distributed in them. Wool fat is also used as an alternative source of cholesterol. It is mainly used to increase the water-absorbing capacity of an ointment base. For example, hydrophilic petroleum U.S.P contains 3% cholesterol, 3% stearyl alcohol, 8% white beeswax and 86% white soft paraffin.

The following are the advantages of absorption bases:

1. The base is easily spreadable and less occlusive in nature
2. They assist in permeation of oil-soluble medicaments into the skin.
3. They possess good emollient property.
4. They are compatible with most of the medicaments.
5. They are relatively thermostable or heat stable.
6. They may be used in their anhydrous form or in an emulsified form.
7. They absorb a large quantity of water or aqueous substances.

Water-miscible bases: These bases are miscible with large amounts of water. Ointments made from such bases are easily removable after use. The three anhydrous water-miscible ointment bases are as follows:

1. Emulsifying ointment B.P (contains anionic emulsifier).
2. Cetrimide emulsifying ointment B.P (contains cationic emulsifier)
3. Cetomacrogol emulsifying ointment B.P (contains nonionic emulsifier)

An example of an ointment containing water-miscible base is compound benzoic acid ointment (Whitfield’s ointment), which is an antifungal ointment.

The following are the advantages of water-miscible bases:

1. Ready miscibility with the exudates from lesions
2. Reduced interference with normal skin function
3. Good adherence to the skin because of their surfactant content
4. Easy removal from the skin and hair
5. High cosmetic acceptability

Water-soluble bases: These bases primarily consist of water-soluble ingredients such as polyethylene glycol (PEG), which are popularly known as “carbowaxes” and commercially known as “macrogols.” Solids can be easily incorporated into these bases without much difficulty. Their water solubility promotes complete removal from the skin. Water-soluble bases are a range of compounds with the general formula CH₂OH(CH₂OCH₂)_nCH₂OH. The PEGs are mixtures of polycondensation products of ethylene oxide and water and are exemplified by numbers representing their average molecular weights. They vary in consistency from viscous liquids to waxy solids. For example, macrogols 200, 300, and 400 are viscous liquids, macrogols 1500 are greasy semisolids and macrogols 3000, 4000, and 6000 are waxy solids. Different grades of PEGs are mixed to get an ointment of desired consistency.

The advantages of PEGs as ointment bases are as follows:

1. They are readily water soluble and, hence, are freely miscible with tissue exudates; they can be easily removed from the skin.
2. They promote good percutaneous absorption.
3. They possess good solvent properties. Some water-soluble dermatological drugs such as salicylic acid, sulfonamides and sulfur are easily soluble in this base.
4. They are nongreasy with good aging property.
5. They neither hydrolyze nor undergo rancidity and hardly support microbial growth.
6. They are compatible with many dermatological medicaments.
7. They can be sterilized by heat and, hence, are preferred for ophthalmic ointments.

The following are their disadvantages:

1. Packaging in plastic containers is difficult due to its remarkable solvent property.
2. There is a reduction in the potency of activity of certain antibacterial agents, such as phenols, hydroxybenzoates and quaternary compounds, when used in PEGs bases.