Disadvantages of Rectal Administration

1. A perceived lack of flexibility regarding dosage of commercially available suppositories in underuse and a lack of availability 2. If made on demand, may be EXPENSIVE 3. Exhibit VARIABLE EFFECTIVENESS 4. Formulations with NARROW THERAPEUTIC MARGIN, cannot be interchanged w/o risk of toxicity 5. Bullet shaped suppository after insertion can leave the anorectal site and ascend to rectostigmoid and descending colon, should not be used at bedtime 6. Defecation may interrupt the absorption process of drug, occur if drug is irritating 7. The absorbing SA of the rectum is much smaller than that of the small intestine 8. Fluid content of the rectum is much less than that of the SI, which may affect dissolution rate 9. Possibility of degradation of some drugs by the microflora present in the rectum 10. Dose of drug may be greater or less than the dose of the oral drug 11. Factors that affect rectal absorption of a drug administered in the form of suppository: a. anatomic and physiologic factors b. physicochemical factors of the drug and the base