Pharmaceutical Ointments and Pastes

Pharmaceutical Ointments and Pastes

• Pharmaceutical ointments are greasy or oily semisolid systems applied externally, primarily to the skin and mucous membranes (e.g., rectum, vagina, eyes).
• Medicated ointments treat infections and inflammation.
• Non-medicated ointments are used for their emollient/lubricating properties.

Pharmaceutical Pastes

• Pastes are semisolid dosage forms with a high proportion of solid components.

• Pastes are thicker and stiffer than ointments.

• Pastes are made from fatty bases (e.g., petrolatum, hydrophilic petrolatum) or aqueous gels (e.g., celluloses).

• They may contain one or more drug substances for topical application.

• Pastes are well absorbed by the skin and are easily removed from the skin.

• Pastes containing hydrophobic components are water-impermeable and prevent dehydration.

• Common examples of pastes include toothpaste and zinc oxide paste.

• Toothpaste contains an abrasive solid for cleaning and sometimes a fluoride salt.

• Pharmaceutical pastes are often composed of ointment bases with a high concentration (often >50% w/w) of dispersed drug.

• Viscosity of pharmaceutical pastes is higher than that of ointments.

Ointment Types

• Dermatological ointments
• Ophthalmic ointments
• Rectal ointments
• Vaginal ointments
• Nasal ointments

Ointment and Paste Bases

• Hydrocarbon/oleaginous bases

  • Formulations are non-aqueous, based on various paraffins.
  • Properties: emollient (restricting water loss), excellent retention on the skin, predominantly hydrophobic (difficult to remove from the skin), low water content (<5%), chemically inert, and provide optimum stability for medications (e.g., antibiotics)
  • Common components: hard paraffin, white/yellow soft paraffin, liquid paraffin (mineral oil), microcrystalline wax

• Absorption bases

  • Can contain significant amounts of an aqueous phase
  • Can be non-aqueous formulations with added aqueous phase (water in oil emulsion; non-emulsified bases) or water in oil emulsions to facilitate the incorporation of the aqueous phase.
  • May contain a larger volume of aqueous phase than hydrophobic bases but are still difficult to remove by washing. Composed typically by one or more paraffins, and sterol-based emulsifying agent (lanolin, lanolin alcohols, bees wax).

• Water-miscible/removable (emulsifying) bases

  • Contain sufficient emulsifying agent (oil in water) that allows them to be miscible with water.
  • These bases are easily washable, not occlusive, and can contain large volumes of water (aqueous solutions of drug).
  • Common bases in this category described by the British Pharmacopoeia include emulsifying ointment, cetrimide emulsifying ointment, and cetomacrogol emulsifying ointment. They comprise liquid paraffin, white soft paraffin, and anionic, cationic, or non-ionic emulsifying wax.

• Water-soluble bases

  • Composed entirely of water-soluble ingredients (e.g., polyethylene glycol).
  • Non-greasy, easily removed via washing, miscible with exudates, compatible with many therapeutic agents.
  • Prepared using mixtures of polyethylene glycols of varying molecular weights to achieve the required consistency (lower weights are liquids and higher weights are waxy solids)
  • Disadvantage: cannot incorporate large volumes of aqueous solutions.

Other Excipients

• Additional/alternative solvents (e.g., liquid silicone, vegetable oils, organic esters)
• Preservatives
• Antioxidants (lipophilic and hydrophilic types)

Selection of Bases

• Ointment base selection factors include desired release rate, occlusion advisability, short-term or long-term stability, drug interaction with base consistency, patient factors (e.g., dry or weeping skin).