http://homepages.paradise.net.nz/alanmcc/Paradise/eMixtdemo/DWoods/Backpages/rectal.html
General Considerations (for rectal administration)
1. Adequate hydration of the dose form is important. If tablets or capsules are being inserted and the rectum is very dry, insert 10 - 15mL of warm water to assist the dissolution or dispersion of the dosage form. A water miscible lubricant such as KY® jelly is useful if tablets or capsules are inserted.
2.
Partial avoidance of hepatic first-pass metabolism. The superior rectal vein, which perfuses the upper part of the rectum, passes directly into the portal circulation. The middle and inferior rectal veins perfuse the lower part of the rectum, and venous blood avoids first pass through the liver. Thus, the positioning of the dose form in the rectum will influence the bioavailability of drugs that are subjected to extensive first-pass metabolism.
3. Large dose volumes are difficult to retain in the rectum and hypertonic solutions have a laxative effect.
4. Some vehicles used for parenteral solutions, such as ethanol and propylene glycol, may be irritant to the rectal mucosa, especially if used chronically.