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Rectal circulation
• Drugs absorbed rectally bypass the portal circulation, thereby enabling drugs that are
destroyed in the liver to exert systemic effects.
• The lower hemorrhoidal veins surrounding the colon receive the absorbed drug and initiate
its circulation throughout the body, bypassing the liver.
• Lymphatic circulation also assists in the absorption of rectally administered drugs.

pH and lack of buffering capacity of the rectal fluids
• Because rectal fluids are essentially neutral in pH and have no effective buffer capacity,
the form in which the drug is administered will not generally be chemically changed by
the environment.

Composition of suppositories

• Active ingredient.

• Additives.

• Suppositories base.

The active ingredient (drug)

Drug properties like drug solubility in the vehicle and vehicle-to-water partition coefficient of the

drug will strongly affect the quality of suppositories.

Drugs with high partition coefficient show a low tendency to leave the vehicle. Consequently, the

release of the drug into the rectal fluid is expected to be low. A balanced partition coefficient is

recommended to allow the release of the drug from the suppository base and absorption through the

rectal membrane for a systemically acting suppository.

Working Example

x

Diazepam powder X mg

Cocoa butter Yg

 Diazepam is fat-soluble as a result; the drug may remain dissolved in the base and may not

release rapidly enough to control the seizures.

 The use of a water-soluble base such as polyethylene glycol may be a better choice.

The suppository base has a marked influence on the release of active constituents. While cocoa

butter melts rapidly at body temperature, because of its immiscibility with fluids, it fails to

release fat-soluble drugs readily.

For systemic drug action using a cocoa butter base, it is preferable to incorporate the ionized

(salt) form rather than the unionized (base) form of a drug to maximize bioavailability. Although

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