When THC or CBD enters your body, it goes on a journey that involves absorption, distribution, receptor binding, metabolism, storage, and elimination — and every step shapes both the effects you feel and how long the compound stays detectable. Here's how each phase actually works.
The endocannabinoid system: your body's home court
Before we talk about THC from a plant, it helps to know that your body is already running a cannabinoid system of its own. The endocannabinoid system (ECS) is a network of receptors, signaling molecules, and enzymes distributed throughout your body. It regulates appetite, sleep, mood, immune response, pain perception, and dozens of other processes.
The system was named after cannabis only because researchers discovered the plant's compounds first; they then realized the body had its own endogenous version. The receptors that THC binds to — primarily CB1 and CB2 — were always there. THC works because it happens to fit, more or less, into a receptor system designed for molecules called anandamide and 2-AG that your body produces naturally.
This is why cannabis effects are so wide-ranging. The ECS touches almost every major bodily system, so a compound that activates it produces effects across many systems simultaneously.
Absorption: getting the compounds in
The first phase is moving cannabinoids from outside your body into your bloodstream. The mechanism depends entirely on how the compound was administered:
From the lungs
Inhaled cannabinoids cross the thin alveolar membranes into pulmonary capillaries within seconds. Roughly 25-50% of the THC in inhaled vapor actually makes it into systemic circulation; the rest is either exhaled or deposited in oral and respiratory tissue. The absorption is fast but not 100% efficient.
From the small intestine
Eaten cannabinoids are absorbed through intestinal walls along with other lipid-soluble substances. THC is highly lipophilic (fat-loving), so it's absorbed more readily when consumed with fat — this is part of why edibles often have fat-based delivery vehicles like coconut oil or butter. Oral bioavailability is much lower than inhalation, typically 4-20% of the dose, with high variability between individuals.
Through mucous membranes
Sublingual administration (tinctures held under the tongue) bypasses the digestive system. Cannabinoids absorb through the rich vascular bed under the tongue directly into circulation. Bioavailability falls between inhalation and oral — typically 10-35% — with onset times in between as well.
Through the skin
Transdermal delivery (designed patches) achieves systemic absorption slowly and at low bioavailability. Topical creams generally don't achieve systemic absorption in meaningful amounts — they act locally on tissues at the application site.
Distribution: where they go in your body
Once in the bloodstream, cannabinoids distribute throughout your body — but not evenly. THC and most other cannabinoids are highly lipophilic, which means they prefer fatty tissue to watery tissue. This has major consequences for both the immediate experience and the long-term clearance.
In the short term, cannabinoids quickly reach tissues with high blood flow and lipid content: the brain (which is roughly 60% fat), the liver, the lungs. This is when you feel the effects.
Over the next minutes to hours, the cannabinoids continue to redistribute. They move from blood plasma into deeper fat stores — adipose tissue, organ fat, etc. — where they remain until slowly released back into circulation. This redistribution is why blood THC levels drop relatively quickly after inhalation even though the total body load remains high.
For occasional users, the fat stores are minor. For chronic users, the cumulative deposit in fat tissue can be substantial. This is the mechanism behind weeks-long urine detection in heavy users: the fat stores slowly release cannabinoids over time, providing a continuous (if low) supply for metabolism and excretion.
Receptor binding: where the effects actually happen
Cannabinoids produce their effects by binding to receptors throughout the body. The two best-characterized cannabinoid receptors are:
CB1 receptors
Densely distributed in the brain, particularly in the hippocampus (memory), cerebellum (coordination), basal ganglia (movement), and prefrontal cortex (executive function). Also present in lower densities throughout the peripheral nervous system. CB1 activation by THC is responsible for most of the psychoactive effects of cannabis: the euphoria, the altered perception, the memory effects, the appetite stimulation, the motor coordination changes.
CB2 receptors
Predominantly distributed in immune cells and peripheral tissues, with some presence in the central nervous system. CB2 activation is associated with anti-inflammatory and immunomodulatory effects, generally without the psychoactive component associated with CB1.
THC is a partial agonist at both CB1 and CB2 — it activates the receptor but not at the full strength your body's own anandamide can produce. CBD is more complicated: it doesn't directly activate either CB1 or CB2 strongly, but appears to modulate the system indirectly, affecting how anandamide and other signaling molecules work.
The full mechanism story includes other receptors too. Some cannabinoid effects appear to involve serotonin receptors (5-HT1A), the GPR55 receptor, peroxisome proliferator-activated receptors (PPARs), and TRP channels involved in pain and temperature sensing. The receptor pharmacology is genuinely complex.
Metabolism: the liver's role
Eventually, every cannabinoid molecule in your body has to be processed for elimination. This processing happens primarily in the liver, by a family of enzymes called the cytochrome P450 system (CYP450).
For THC specifically, the main metabolic enzyme is CYP2C9, with secondary contributions from CYP3A4. CYP2C9 converts THC into 11-hydroxy-THC — a metabolite that's actually more potent than the original molecule and crosses the blood-brain barrier more efficiently.
If you inhaled the THC, most of it reaches the brain before the liver gets a meaningful chance to metabolize it. You feel the effects from THC itself.
If you ate the THC, it has to pass through the liver before reaching general circulation (this is called "first-pass metabolism"). A substantial fraction gets converted to 11-hydroxy-THC during that first pass. By the time the cannabinoids reach your brain, the mix is heavily weighted toward the metabolite rather than the original compound. This is why edibles feel different from smoking — different molecule doing different work.
From 11-hydroxy-THC, the liver continues processing toward THC-COOH (11-nor-9-carboxy-THC), an inactive metabolite that's the actual marker most drug tests look for. THC-COOH is highly lipophilic and accumulates in fat tissue, contributing to the long detection windows in chronic users.
Elimination: how it leaves
The cannabinoid metabolites are eventually excreted via two main routes:
- Feces (60-65%): Most THC metabolites are eliminated through the digestive tract via bile. The liver dumps metabolites into bile, which enters the small intestine, and from there exits the body.
- Urine (25-30%): A smaller portion is filtered through the kidneys and excreted in urine. The metabolites in urine are primarily THC-COOH and its glucuronide conjugates — and these are what standard urine drug tests detect.
- Other minor routes: Small amounts via sweat, hair (where cannabinoids get incorporated into the follicle during growth), and even saliva.
The half-life of THC and its metabolites varies enormously by individual and by how the compound was used:
- THC in plasma after single use: Half-life roughly 1-2 days
- THC-COOH in urine after single use: Detectable for 3-7 days
- THC-COOH in urine after chronic heavy use: Detectable for 30+ days, occasionally much longer
Why all of this matters
Understanding the full pharmacological pathway answers questions that come up constantly:
Why does THCA flower fail drug tests even when sold as hemp? Because once heated, THCA becomes THC, which becomes THC-COOH, which is what drug tests detect. Legality at point of sale doesn't change the metabolic pathway. (Note that the federal hemp definition itself is changing as of November 12, 2026 under Public Law 119-37, which adopts a total-THC standard that counts THCA.)
Why do edibles feel different from smoking? Because oral consumption produces 11-hydroxy-THC as the dominant active compound rather than THC itself. Different molecule, different effects.
Why do heavier users test positive for longer? Because cannabinoids accumulate in fat tissue and slowly release over time. The more body fat and the more cumulative use, the longer the elimination tail.
Why does CBD feel different from THC? Because CBD doesn't strongly activate CB1 or CB2 receptors the way THC does. The effects come from indirect modulation of the endocannabinoid system rather than direct receptor activation.
Individual variation
One thing the textbook pharmacology doesn't capture well is how much individual variation exists in cannabinoid response. Several factors contribute:
Endocannabinoid baseline. People have different baseline levels of endogenous anandamide and 2-AG, different receptor densities in different brain regions, and different breakdown enzyme activity. Two people with identical THC blood levels can have notably different subjective experiences because their underlying endocannabinoid systems are operating from different baselines.
CYP enzyme variants. Genetic variation in the cytochrome P450 enzymes that metabolize THC means some people convert THC to 11-hydroxy-THC much faster than others. This particularly affects edible response.
Body composition. Fat mass affects storage and elimination. Lean mass affects distribution volume. Hydration status affects renal clearance of metabolites.
Tolerance state. Regular users develop receptor downregulation that significantly shifts dose-response curves. The same dose that incapacitates a naive user might be barely noticeable to a chronic daily user.
Age and other medications. Liver function declines with age, slowing metabolism. Other medications metabolized by the same CYP enzymes can compete for processing capacity, altering effective exposure to either substance.
This is why "5mg of THC" doesn't produce a uniform experience across consumers. The pharmacology is consistent at the molecular level; the human variation that sits on top of it is substantial.
The pharmacology isn't just academic. It explains the practical experience of cannabis use in ways that pure cultural knowledge can't. Knowing what your body actually does with these molecules is the foundation for understanding why different products produce different effects, why detection windows vary so much, and why no two people respond identically even to the same dose.
