Semi-solid dosage forms are pharmaceutical formulations that have a consistency between solids and liquids. They are used for external application to the skin or mucous membranes, including ointments, creams, pastes, and gels.

According to the United States Pharmacopeia (USP), ointment bases are classified into four groups:

• Oleaginous bases
• Absorption bases
• Water-removable bases
• Water-soluble bases

An oleaginous base is a type of ointment base that consists primarily of hydrocarbons, such as petrolatum. These bases provide a greasy effect and create an occlusive barrier on the skin.

Absorption bases can absorb water to form emulsions. Examples include wool fat and hydrous wool fat.

Penetration enhancers, such as dimethyl sulfoxide (DMSO), alcohol, and urea, help increase drug absorption through the skin.

Ointments contain fewer solid particles and are used for occlusive and emollient effects.

Pastes have a higher content of solid particles and are used for protective applications.

Glycerin is incorporated into vanishing creams to prevent drying and to maintain skin hydration.

Cold cream bases can be either water-in-oil (w/o) emulsions or oil-in-water (o/w) emulsions, depending on their formulation.

Whitefield’s ointment contains benzoic acid and salicylic acid as its main active ingredients.

Petrolatum and white petrolatum are used as oleaginous bases due to their occlusive nature, which helps retain moisture in the skin.

Oleaginous bases remain on the skin for extended periods and are difficult to remove with water.

Polyethylene Glycol (PEG) is an example of a water-soluble base that allows easy washing and does not leave a greasy residue.

Common equipment includes:

• Roller mill – for mixing and refining
• Chilsonator – for compacting powder ingredients
• Grinding mill – for reducing particle size

Triethanolamine (TEA) is commonly used to neutralize carbomer and form a gel.