What is bioavailability?
Bioavailability is a term that is crucial to the understanding of pharmacology, which refers to “the degree and rate at which an administered drug is absorbed by the body’s circulatory system, the systemic circulation.”
When trying to understand how any drug, and medical cannabis specifically, is absorbed and metabolised by the body, understanding bioavailability is key. Essentially, bioavailability determines the efficacy of drugs by giving us an idea of the extent to which and how quickly the medical cannabis taken becomes active at the intended site of physiological activity.
The baseline of bioavailability for any substance is intravenous injection. Any drug that is taken in this way is 100% bioavailable as the substance is immediately absorbed into the bloodstream. However, there are currently no intravenous formulations of medical cannabis, so instead, the medical cannabis being taken by patients has to bypass various biological hurdles depending upon the route of administration used.
So, to understand the effectiveness or bioavailability of cannabis-based medicines, we need to consider various routes of administration and how this affects the absorption of the medicine.
Routes of administration
Cannabis medicines can be taken in a number of ways, with cannabis inhalation, consumption and topical application all being possible. Respectively, this generally means smoking or vaping, eating edible products or pure oils, and rubbing salves or balms into the skin.
The relative bioavailability of all these different options is quite variable.
Inhaling cannabis has a relatively high degree of bioavailability when compared to other administration options. In a study from 2005 called ‘Pharmacokinetics of cannabinoids’, it was found that smoked THC had about 30% average bioavailability, however it should be noted that it’s much harder to control the dosage when dealing with a smoked substance than it is when vaping, as vapes tend to have temperature controls that can help with this process. And while smoking may exhibit a high degree of bioavailability, smoking is not typically clinically recommended due to the potential for respiratory harms linked with this administration method.
Aside from smoking cannabis in a joint, vaping also exhibits a high degree of bioavailability. Indeed, a 2016 study, ‘Medicinal Cannabis: In Vitro Validation of Vaporizers for the Smoke-Free Inhalation of Cannabis’, identified that with the right choice of vaporizer, the bioavailability of THC and CBD can reach percentages as high as 50 to 80%. However, again caution should be advised when seeking out vaping administration methods, as with the ‘vaping crisis’ experienced in the US last year, the safety of vaping has somewhat been thrown into question.
We recommend, as we recommend with all cannabis administration methods, that consultations with trained medical professionals are carried out so that medical administration can be monitored and controlled following a full medical assessment.
Cannabis edibles can be hugely variable in terms of bioavailability. This is because cannabinoids are lipophilic, so if they aren’t combined with a fatty substance when consumed, bioavailability will be significantly limited. Additionally, the ‘first-pass effect’ means that often when consuming edibles, the liver will inhibit the absorption of the cannabinoids.
With all this considered, the common consensus amongst a number of studies on edibles is that their typical bioavailability falls between 4 and 20%. This clearly makes edibles much less efficient than inhaled routes of administration.
Topical applications tend to have little impact with any internal medical conditions as the skin itself acts as a barrier to absorption.