Rectum bypass

Unlike the small intestine and upper colon, the vasculature draining the rectal cavity does not totally direct the blood supply to the liver. The lower and middle hemorrhoidal veins of the rectum bypass, at least... 

Drugs absorbed from the lower part of the rectum bypass the liver. 

Rectal Administration Rectal administration of a drug may be applied when the patient is unable to take the drug orally and some other routes are impractical. The drug administered via the rectum is absorbed and partially bypasses the liver. However, the absorption of drugs may be unreliable in certain cases. 

The hepatic first-pass effect can be avoided to a great extent by use of sublingual tablets and transdermal preparations and to a lesser extent by use of rectal suppositories. Sublingual absorption provides direct access to systemic—not portal—veins. The transdermal route offers the same advantage. Drugs absorbed from suppositories in the lower rectum enter vessels that drain into the inferior vena cava, thus bypassing the liver. However, suppositories tend to move upward in the rectum into a region where veins that lead to the liver predominate. Thus, only about 50% of a rectal dose can be assumed to bypass the liver. 

FIGURE 7.1 Schematic illustration of the venous drainage of the human rectum. Drugs absorbed in the inferior and middle rectal veins that drain the lower part of the rectum will be delivered preferentially to the systemic circulation, bypassing the liver and avoiding first-pass metabolism. (Illustrated with modification from Nishimura, M., et al. (Eds.), Shin-Gekakgaku, 5th ed., Nanzando, Tokyo, 1979, 583. With permission.)... 

Archeological evidence of psilocybin liquid via enemas dates back to A.D. 1 and some tribes still practice this method today. In ancient times, a hollow bone or tube was inserted deep into the rectum. Then an animal bladder filled with the psilocybin liquid was attached to the end of the tube. The liquid was then squirted deep into the rectum and lower intestine. Because so many natural barriers are bypassed by this method, the user is at great risk for overdose, serious side effects, or death. 

In addition to its application in the subcutaneous controlled administration of drugs for pharmacological studies, this technology has recently been extended to the controlled administration of drugs in the rectum by zero-order kinetics. The hepatic first-pass metabolism of drugs is thus bypassed. ... 

Suppositories may slip into the rectum and melt there, bypassing the anal areas where the medication is needed and increasing the possibility of systemic absorption of local anaesthetics and hydrocortisone. This possibility is increased if the patient is lying down. 

The rectal route, though less predictable, can be used when oral ingestion is precluded because the patient is unconscious or when vomiting is present. Approximately 50Vo of the drug that is absorbed from the rectum will bypass the liver, thus reducing the hepatic first-pass effect. 

---