Rectal Administration

Some drugs are administered rectally either in suppository or in solution form, e.g., retention enema. The solution yield better absorption provided that they are retained for a sufficient length of time in the rectum. The suppositories are the most commonly used dosage forms for both local and systemic effect. Examples of drugs administered rectally for systemic action include aspirin, acetaminophen, indomethacin, diazepam, theophylline, prochlorperazine, cyclizine, promethazine, and barbiturates.

The absorption mechanism mainly involves passive diffusion with no sites for active transport. The absorption rate and bioavailability are more erratic than observed with oral administration, due to such added factors as the presence of feces retarding absorption or irritant suppository bases such as carbowaxes causing early evacuation. The use of an enema before drug administration generally increase the absorption significantly.

TABLE 6 Drugs for Which First Pass Hepatic Biotransformation Is Suspected, Possibly in Addition to Gastrointestinal Biotransformation

Alprenolol

Pheniprazine

Desmethylimipramine

Propranolol

Dopamine

Reserpine

Lidocaine

Serotonin

Nortriptyline

Tryptophan

Oxyphenbutazone

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