The Medicinal Punch of THC Depends on How You Take It - RxLeaf

The Medicinal Punch of THC Depends on How You Take It

Christine Kielhorn PHD
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Inhaling, sublingual, rectal, oral: how you take cannabis will determine how effective it is at treating your medical condition; because the bioavailability changes.

As with any medicine, understanding pharmacokinetics and bioavailability of cannabis are critical to knowing how to use it effectively, particularly for applications in which a high or very specific dose is required.

Pharmacokinetics is the study of how a drug moves through the body, as it attempts to define and predict how the body will absorb, distribute, metabolize, and excrete the drug and how long these processes take. Pharmacokinetics helps scientists and physicians to determine what an appropriate dose of medicine should be for a given body sizeand 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, or ways you can take it, and each of these will have slightly different pharmacokinetics.

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The pharmacokinetics depends on the drug’s bioavailability which will differ with each route of administration 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. The “absolute bioavailability” is calculated 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 100% bioavailability as a reference.

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The “relative bioavailability” compares the amount of drug in the bloodstream between two different routes of administration, neither of which being IV injection. Losses of drug that would reduce its biovailability are usually caused by first-pass metabolization in the liver, exhalation, limited absorption, and conversion into inactive metabolites by enzymes in the body. If high doses of a drug are needed, 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.

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The most common routes of administration of THC are inhalation (via smoking or vaporizing), oral, and oromucosal (in the form of sprays or sublingual tinctures). However, rectal suppositories and injections are also possible, though less common. Here is a breakdown of the relative bioavailability of THC in each of these uses.


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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 20% (studies have reported between 23-27%). Those who are considered “light” users of Cannabis have lower inhalation availability, closer to 10-15%. However, the formulation does matter – a study on the use of a nebulizer for the vaporization of THC found that bioavailability was 28% in those who were not Cannabis smokers.

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Oral and Sublingual

Oral route has a lower bioavailability than inhalation thanks to first-pass elimination of THC by the liver. When THC is swallowed, whether in the form of a pill or an edible, the drug absorbed into the bloodstream goes straight to the liver where it is converted into other inactive metabolites. Most studies of oral biovailability estimate 6% of THC will make it through the liver into general blood circulation. In addition, the time it takes for THC concentrations to peak in the blood is much longer than other routes of administration. However, because the oral administration route is familiar and the easiest for patients to swallow, tweaking the formulation to improve the bioavailability and pharmacokinetics of orally dosed THC has yielded promising results. One clinical study, on a THC formulation called Namisol, found that the oral dose had a relatively short time to reach maximum blood concentrations (about 1 hr) compared to other oral THC drugs.

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Another study 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.

The sublingual route of administration means the drug is applied under the tongue, where it is absorbed 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 comparing the oromucosal route with an oral dose of THC found that while the bioavailability of THC using the oromucosal spray was slightly higher, the difference was not statistically significant, and so the bioavailability of oromucosal spray is approximately the same as oral THC.


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Parenteral injections, whether intravenous (IV) or intramuscular (IM), have also been evaluated as potential routes of administration of THC. They would obviously have the highest bioavailability at 100%, theoretically. An early study on rhesus monkeys found that IM injections had a high bioavailability of 102%, and it 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 12%. A study of rectal suppositories in rhesus monkeys found that bioavailability was approximately 13.5%.

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 bioavailability. Rectal suppositories are the next highest, and oral and oromucosal have the lowest.


Christine Kielhorn
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