Be it inhalation, sublingual, oral or rectal, the bioavailability of THC changes with each consumption method.
As with any medicine, understanding the pharmacokinetics and bioavailability of THC are critical. These help us to know how to consume it effectively. All the more so for applications in which we require a high or specific dose.
Pharmacokinetics is the study of how a drug moves through the body. It attempts to define and predict how the body will absorb, distribute, metabolize, and excrete the drug. But also how long these processes take. Pharmacokinetics helps scientists and physicians to determine what an appropriate dose of medicine should be for each given body size, and how often it should be taken in order to maintain therapeutic levels in your body. With delta-9-tetrahydrocannabinol (THC), there are several common routes of administration. And each of these will have slightly different pharmacokinetics which means the bioavailability of THC for each consumption method differs.
Pharmacokinetics and Bioavailability
The pharmacokinetics depends on the drug’s bioavailability which will differ with each consumption method and the formulation used (for example, if it is mixed with something to make a pill). The bioavailability refers to the fraction of the total dose that actually reaches its intended target. Calculating the “absolute bioavailability” occurs by comparing the concentration of the drug that makes it into the bloodstream to the concentration after an intravenous (IV) injection. An IV injection provides a theoretical one hundred percent bioavailability as a reference.
The “relative bioavailability” compares the amount of drug in the bloodstream between two different routes of administration. Neither of these were IV injection. Losses of a drug that would reduce its availability usually come from first-pass metabolization in the liver. Or, indeed, also exhalation, limited absorption, and conversion into inactive metabolites by enzymes in the body. If you need high doses of a drug — as is the case with some medicinal applications of THC — the bioavailability of the drug is important to consider to maximize the efficiency of each dose.
The most common routes to administer THC are:
- inhaling (via smoking or vaporizing),
- and oromucosally (in the form of sprays or sublingual tinctures).
However, rectal suppositories and injections also work, though are less common. Here is a breakdown of the relative bioavailability of THC in each of these consumption methods.
The pharmacokinetics and bioavailability of inhalation are the most studied because it is the most commonly used route of administration, both medicinally and recreationally. Overall, inhalation provides the greatest bioavailability of the three most common approaches. Several studies have measured the blood concentrations of THC in people after they smoke (or otherwise inhale) a controlled dose of THC.
These studies have found that the absolute bioavailability of THC depends on the individual and their history. Those who use Cannabis frequently had a higher bioavailability of THC – greater than twenty percent (studies have reported between twenty three and twenty seven percent). Those who studies consider “light” consumers of cannabis have lower inhalation availability, closer to ten to fifteen percent. However, the formulation does matter — a study on the use of a nebulizer for the vaporization of THC found that bioavailability was twenty eight percent in those who were not cannabis smokers.
Oral route has a lower bioavailability than inhalation thanks to first-pass elimination of THC by the liver. When you swallow THC, as pill or edible, the drug absorbs into the bloodstream and goes straight to the liver. From there our body converts it into other inactive metabolites. Most studies of oral biovailability estimate six percent of THC will make it through the liver into general blood circulation.
Further, THC concentrations peaking in the blood take much longer than other routes of administration. However, oral administration is the most familiar and the easiest for patients to swallow. Thereby, tweaking the formulation to improve the bioavailability and pharmacokinetics of oral THC yields promising results. One clinical study, — published in the British Journal of Clinical Pharmacology (2012) — on a THC formulation called Namisol, found that the oral dose had a relatively short time to reach maximum blood concentrations (about one hour) compared to other oral THC drugs.
Another study — published in Cancer Chemotherapy and Pharmacology (2017) — found that dronabinol (a synthetic form of THC) formulated in a solution had a higher bioavailability and better pharmacokinetics than dronabinol tablets. A formulation of THC that utilizes liposomes, particles, or other emulsification techniques could also improve bioavailability by protecting it from elimination by the liver.
This route of administration means you apply the drug under the tongue. From there, it absorbs into the bloodstream through the oromucosal membranes. This route is similar to other oromucosal applications like sprays. In theory, this approach should provide higher bioavailability. Because THC is essentially delivered directly to the bloodstream and avoids first-pass metabolism of the liver.
However, it does rely on the absorption of THC through these membranes, which varies from patient to patient and is dependent on other factors, such as whether or not the patient has recently eaten. One study — published by the American Association of Clinical Chemists (2010) — comparing the oromucosal route with an oral dose of THC found that the bioavailability of THC using the oromucosal spray was slightly higher. However, the difference was not statistically significant, and so the bioavailability of oromucosal spray is approximately the same as oral THC.
Bioavailability of Other Techniques
Parenteral injections, whether intravenous (IV) or intramuscular (IM), are also seeing study as potential routes of administration for THC. They would obviously have the highest bioavailability at one hundred percent, theoretically. An early study on rhesus monkeys found that IM injections had a high availability of 102 percent. This was also the route with the shortest time to maximum blood concentrations when compared to oral administration.
Rectal suppositories are estimated to have a bioavailability twice that of oral doses, so approximately twelve percent. A study of rectal suppositories in rhesus monkeys, published in the Journal of Pharmaceutical Science (1991), found that bioavailability was approximately 13.5 percent.
Bioavailability of THC determines how much of the THC you take actually makes it to your blood stream. Apart from injections, inhalation has the highest availability. Rectal suppositories are the next highest, and oral and oromucosal have the lowest. Knowing this absorption rates