there are tons of questions here about rectal administration of drugs.
i would be very wary of rectal admin of anything unless you have found a medical resource showing it's efficacy.
the fact is there are a lot of factors that are important to consider:
- some drugs have been shown to absorb slower than oral admin, some faster. the rate of absorption for most drugs is not known.
- volume of the drug is going to impact the delivery route too. a small suppository or small amount of liquid will indeed enter the bloodstream directly (and bypass the liver) but a larger amount will enter a upper area of the rectum or colon and will enter the liver and be subject to it's metabolism
from some medical sources:
Pharmacokinetics of rectal drug administration, Part I. General considerations and clinical applications of centrally acting drugs.
van Hoogdalem E1, de Boer AG, Breimer DD. (PubMed):
"The rate and extent of rectal drug absorption are often lower than with oral absorption, possibly an inherent factor owing to the relatively small surface area available for drug uptake"
Drug Absorption by Sublingual and Rectal Routes
A. G. De Boer, L. G. J. De Leede And D. D. Breimer
The disadvantages associated with administration of drugs rectally include:
(a) interruption of absorption by defaccation, which may occur particularly with irritant drugs;
(b) the surface area of the rectum is far smaller for absorption than that of the duodenum;
(c) the fluid contents of the rectum are much smaller than those of the duodenum and this may produce problems with dissolution of some drugs;
(d) degradation of some drugs by micro-organisms may occur in the rectum;
Rectal drug administration: clinical pharmacokinetic considerations.
de Boer AG, Moolenaar F, de Leede LG, Breimer DD.
"Only limited number of cases has it been adequately shown that the rectal route of administration gives plasma concentrations which are comparable to the oral route."
Exploring the Unconventional Routes — Rectal and Vaginal Dosage Formulations
The Pharmaceutical Journal (A Royal Pharmaceutical Society publication):
"However, drug delivery via the rectal cavity can be challenged by erratic drug absorption due to potential expulsion of the dosage form and low adhesion to the rectal membrane. In addition, drug bioavailability is affected by the site of drug absorption within the rectal cavity. The superior rectal veins go via the liver whereas the inferior rectal veins bypass liver metabolism, thus correct positioning of the dosage form, at the lower rectum, is crucial."
if you are considering this with your method please research it carefully.
