AbsorptionThis is an early phase of drug pharmacokinetics. Absorption of a cannabis compound, such as THC, refers to compound’s entering into the bloodstream. Having a drug in your mouth may seem like it’s in your body, but it needs to enter the blood first before it’s considered absorbed. There are various ways for THC to enter the bloodstream, and each way has its own benefits and challenges. These ways, called administration methods, can be intravenous, i.e. (drug injection directly into the vein) or extravascular, for example, administration via the mouth (oral), the mouth tissue (oromucosal), the skin (transdermal), the anus (rectal), (), inhalation via the lungs (intrapulmonary), etc. In general, THC is almost instantly absorbed after inhalation, within minutes. In contrast, after oral ingestion, it can take up to an hour or more before a large amount of the THC is absorbed from the gastrointestinal tract.1, 2, 3, 4 Illustration: Absorption via extravascular system – If a drug is given by any other route, it is considered extravascular and must thefore undergo absorption. Absorption is a first order of process. As more drug reaches the absorption site, the more is absorbed (by a constant fraction). However, as drug enters the cirsculation, it is also eliminated. At some point, the absolute amount of drug offered to the systemic circulation is diminishing (because most of it is now absorbed). This causes a flattening of the concentration time curve and it will reach maximum concentration called Cmax at time tmax. At this point the rate at which the drug enters the circulation through clearance and this is analogous to an intravenous infusion. It is also possible that not all of the drug reaches the systemic circulation.
Illustration caption and image taken with permission from TRC Pharmacology Database.References:
- Grotenhermen, Franjo (2003). Clinical Pharmacokinetics of Cannabinoids. Journal of Cannabis Therapeutics, 3(1), 3--51.
- Grotenhermen, Franjo (2003). Pharmacokinetics and pharmacodynamics of Cannabinoids (327--360).
- Grotenhermen, Franjo (2004). Clinical Pharmacodynamics of Cannabinoids. Journal of Cannabis Therapeutics, 4(1), 29--78.
- Grotenhermen, Franjo.; Russo, Ethan. (2002). Cannabis and cannabinoids : pharmacology, toxicology, and therapeutic potential. Haworth Integrative Healing Press.