Jeffrey Hergenrather, MD, in response to a question from a cancer patient’s caregiver,  February 7, 2015

I have checked the literature to see if there are any pharmacodynamic/pharmacokinetic studies on cannabinoid absorption via the rectum.  There aren’t to my knowledge.  So far I only have anecdotes as you’ve mentioned.  I’m more confident about the oral route just because I have much experience with that but I wouldn’t discount the stories of efficacy by rectal route.  

There is a “watershed” of veins in the rectum.  Collectively they are called the rectal or hemorrhoidal plexus.  The majority of the venous flow ( after absorbing cannabinoids ) goes by way of the superior rectal vein into the portal vein, while the inferior veins flow into the vena cava.  The portal vein collects most all venous return from the pancreas to the rectum with all the nutrients and enzymes for digestion) where the blood goes on to the liver for what is called “first-pass metabolism.”  The majority of the THC is converted to 11-OH-THC, a similarly psychoactive molecule.  The vena cava on the other hand goes back to the heart and general circulation before it more slowly reaches the liver for metabolism.  

I don’t mind sharing a link for a recent advert for suppositories, it is just that I don’t believe much of the information.

I question many of the allegations in this article as unsupported by controlled laboratory evidence.  Particularly in the statements saying that the absorption is much faster and more complete than the ingested route.  I agree that the inhaled route is only about 10-20% absorbed.  Oral route varies widely depending on meals and type of preparations.  Cannabis leaf is not well absorbed whereas oils are absorbed quite well.  I would expect large meals would retard absorption compared to small oily meals.  I don’t know where the “50-70% efficiency” or the “10-15 minute onset” information comes from other than well intended anecdotes.  

 The key point about rectal administration is to avoid psycho-activity while achieving high-dose cannabinoid therapy.  If I were striving for high-dose administration I would use the oral route until more evidence reveals the dynamics of the rectal route.  

I suspect cannabis clinicians tend to recommend more cannabis /cannabinoids much of the time as the urgency of the clinical course demands.  When there is more time I suspect small steady doses are adequate for the sensitive cancers whereas the less sensitive cancers may require the megadoses of oil, “60 grams over 90 days”, or, 20-25 mg/kg/day of cannabinoids irregardless of the urgency.  Eventually, clinical studies will guide our recommendations.  Until then try to get as much cannabinoids into the body by any route, with frequency of administration at least daily, and preferably a few times per day.

 Again, best wishes,

Jeff Hergenrather MD

A follow-up from Dr. Hergenrather is here.