THC Metabolism: Complete Pharmacokinetic Science Guide

Absorption: Inhalation vs Oral Routes

Inhaled THC reaches peak plasma concentrations within 3-10 minutes of smoking, with bioavailability ranging from 10-35% depending on inhalation depth, breath-hold duration, and product potency. The rapid absorption via pulmonary capillaries bypasses first-pass hepatic metabolism, explaining the fast onset and predictable dose-response of smoked or vaporized cannabis.

Oral THC absorption is dramatically different. Gastrointestinal absorption is slow (peak plasma at 1-2 hours) and highly variable between individuals (5-20% bioavailability). Critically, oral THC undergoes extensive first-pass metabolism in the liver, converting much of the absorbed THC directly to 11-OH-THC before reaching systemic circulation. This explains why edibles produce stronger, longer-lasting, and more unpredictable effects than inhalation at the same dose.

Sublingual administration (as used in cannabis tinctures and Sativex) achieves intermediate bioavailability (13-19%) with faster onset than oral ingestion. Understanding THC basics helps contextualize why route of administration matters so profoundly for patient experience. The cannabis pharmacokinetics overview covers all routes in detail.

Hepatic Metabolism: CYP2C9, CYP3A4, and Active Metabolites

The liver is the primary site of THC biotransformation. Cytochrome P450 enzymes, principally CYP2C9 and CYP3A4, hydroxylate THC at the 11-position to produce 11-hydroxy-THC (11-OH-THC), the primary active metabolite. 11-OH-THC crosses the blood-brain barrier more efficiently than THC itself and is estimated to be 1.5-7x more potent at CB1 receptors, contributing significantly to edible effects.

Subsequently, 11-OH-THC is oxidized to 11-nor-9-carboxy-THC (THC-COOH), an inactive metabolite that undergoes glucuronidation for renal excretion. THC-COOH is the primary target of urine drug screens, detectable at nanogram-per-milliliter concentrations days to weeks after last use.

Genetic polymorphisms in CYP2C9 (the poor metabolizer phenotype, affecting 2-3% of the population) result in significantly slower THC clearance, prolonged plasma half-life, and potentially greater cognitive impairment. Cannabis drug interaction research is critical here, as CYP2C9 inhibitors like fluconazole can dramatically elevate THC plasma levels when co-administered.

Distribution and Lipophilicity

THC is highly lipophilic (log P approximately 6.97), meaning it distributes extensively into fat tissue, brain, liver, and lung. The apparent volume of distribution is approximately 10 L/kg, explaining why THC persists in fat stores long after psychoactive effects have resolved.

This lipophilicity has critical implications for drug testing: THC is slowly released from adipose tissue back into circulation, and chronic heavy users can test positive on urine screens for up to 30 days after cessation. Single-use individuals typically clear detectable THC-COOH within 3-4 days.

Plasma protein binding (primarily to albumin and lipoproteins) is approximately 97%, meaning only a small fraction of circulating THC is pharmacologically active. This high protein binding also limits hemodialysis effectiveness in cases of cannabis overconsumption, an important consideration for emergency physicians managing cannabis hyperemesis syndrome or accidental pediatric ingestions.

Elimination and Detection Windows

THC elimination follows multi-compartmental pharmacokinetics: an initial rapid distribution phase (t1/2 approximately 4 minutes), followed by a slower redistribution phase from fat stores, and a terminal elimination half-life of 20-36 hours in infrequent users and up to 13 days in chronic heavy users.

Fecal excretion accounts for approximately 65% of THC elimination (as metabolites), with renal excretion contributing 20-30%. Biliary recycling (enterohepatic circulation) extends the elimination timeline by allowing intestinally secreted metabolites to be reabsorbed.

Detection windows vary significantly by matrix: blood (1-12 hours active THC; 1-25 days THC-COOH), oral fluid (4-72 hours), urine (3 days occasional use; 25-30 days chronic use), and hair (90 days per 1.5cm segment). Understanding these windows is relevant for both workplace drug testing policy and for withdrawal research timelines. The relationship between plasma THC and impairment is poor, complicating roadside cannabis testing policy development.