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un-ionized drugs more readily partition out of water-miscible bases such as glycerinated gelatin
and polyethylene glycol, the bases themselves tend to dissolve slowly and thus retard the release
of the drug.
Additives
Additives are added to suppositories formulation to improve the quality of suppositories such as:
• Plasticizer: e.g. cetyl alc., propylene glycol
• Antioxidant
• Dispersant
• Surfactants may be used as absorption enhancers, wetting agents, or deglomerators which

may prevent the formation of cake in the melting suppositories.
Surfactants in a concentration higher than the critical micelle concentration (cmc) can retard
drug release from the suppositories.
• Viscosity imparting agents: e.g. silicon oxide or aluminium monostearate. They create a gel-
like system with a slower release rate of the drug.

Suppository bases (the vehicle)
They act as a vehicle for medication.
The ideal properties of suppository bases are:
• Melt at 37C to melt after insertion in the body or dissolve and mix with the available volume
of rectal fluid. The melting range should be small enough to rapid solidification after
preparation; thereby preventing the separation of the suspended drug when the solidification
rate is high this may result in fissures, especially when rapid cooling is applied.
On the other hand, the melting range should be large enough to permit easy preparation.
• The base should exhibit enough volume reduction during solidification to permit removal
from the mold.
• Allow optimum drug release.
• The viscosity of the molten base should be suitable to avoid difficulties during
manufacturing (viscosity determines the flow of base into the mold and the rate of particles
separation) and allow reasonable diffusion and release of the drug from the vehicle.
• Non absorbable/ non irritant.
• Compatible with drugs.
• Easy handling and economic.
• Chemically and physically stable during storage and after preparation.

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