THC Pharmacokinetics — How Cannabis Moves Through the Body

THC is extremely lipophilic (fat-soluble). This single property is the most important driver of everything about cannabis drug testing — including why chronic users test positive for weeks, why body composition matters so much, and why no product can safely accelerate clearance.

Absorption — How THC Enters the Body

THC bioavailability depends dramatically on the route of administration:

Inhaled (smoked/vaped): 10–35% bioavailability. Peak plasma concentrations within 6–10 minutes. Rapid onset of effects.
Oral (edibles): 4–12% bioavailability. First-pass hepatic metabolism reduces total THC reaching systemic circulation, but produces proportionally more 11-OH-THC (which is also psychoactive). Onset 30–120 minutes; duration 4–12 hours.
Sublingual/oromucosal: Bypasses first-pass metabolism. Bioavailability varies widely.
Topical: Minimal systemic absorption for non-THC cannabinoids. Transdermal patches can achieve meaningful systemic concentrations.

Distribution — Where THC Goes

Once in the bloodstream, THC rapidly distributes into lipid-rich tissues: adipose tissue, brain, liver, lungs, and other organs with high fat content. This is not a metaphor — THC physically partitions out of blood and into fat cells where it can remain for weeks.

Johansson et al. (1989) reported that THC was detectable in human fat biopsies 28 days after exposure. The fat storage compartment acts as a slow-release reservoir: THC leaves the bloodstream quickly at first (as it partitions into fat), then re-enters the bloodstream slowly as fat tissue releases it.

This is the central fact of THC pharmacokinetics. A single dose of THC does not simply enter your bloodstream, get metabolized, and leave. It distributes into fat, sits there, and slowly re-enters circulation over days to weeks. In chronic users, repeated doses build up a substantial "body burden" that can take months to fully clear.

Half-Life Data

Half-life estimates for THC and THC-COOH vary by study, population, and duration of monitoring. The key distinction is between occasional and chronic users:

Parameter	Occasional Users	Chronic Users
THC plasma half-life	~1–3 days	~5–13 days
THC-COOH urinary half-life (7-day monitoring)	31.5 ± 1.0 hours	Extended
THC-COOH urinary half-life (14-day monitoring)	44–60 hours	Extended further
Terminal elimination half-life (chronic heavy users)	—	9.6–12.6 days

Why does the half-life increase as monitoring extends? Because the slow-release tail from fat tissue becomes more apparent over longer observation windows. Early in the observation, you are watching the fast-clearing blood compartment. Later, you are watching the slow-leaching fat compartment. The fat compartment dominates the terminal phase.

Huestis MA, Cone EJ. "Urinary excretion half-life of 11-nor-9-carboxy-delta9-tetrahydrocannabinol in humans." Therapeutic Drug Monitoring 1998;20(5):570-576.. PMID: 9780137

Elimination — How THC Leaves the Body

Most of a cannabis dose never leaves through urine. Elimination distributes roughly as follows:

~65% excreted in feces (via biliary excretion after hepatic metabolism)
~20% excreted in urine (primarily as THC-COOH-glucuronide)
Small amounts in sweat, saliva, and breast milk

This matters for testing strategy: urine tests capture only about 20% of total metabolite elimination, but it is the 20% that is easiest to collect and quantify. Fecal testing would theoretically provide a more complete picture but is not practical for routine workplace use.

Why Chronic Users Test Positive for Weeks

Chronic cannabis use creates a massive THC body burden in adipose tissue. With each dose, more THC deposits into fat. At steady state, daily users may have THC stored in fat at concentrations that will take weeks to release back into circulation once use stops.

The Bergamaschi et al. 2013 study (PMID 23449702) found cannabinoids remained detectable in blood of chronic daily smokers during up to 30 days of sustained, monitored abstinence. 16 of 25 subjects (64%) had at least one positive blood THC ≥0.25 ng/mL during the 7-day monitoring period. In a separate study of chronic adolescent and young adult users, Schuster et al. (2020) found that 40% still had THC-COOH concentrations above 5 ng/mL after 25+ days of verified abstinence.

This is not a failure of willpower or a technical error. It is the direct pharmacological consequence of how cannabis is stored in the body.

Variables That Affect Detection Windows

Body fat percentage — higher body fat means more THC storage capacity and longer detection windows. Exercise-induced plasma THC increases correlate positively with BMI (r = 0.57).
Frequency and chronicity of use — by far the biggest variable. Single-use vs. daily heavy use can differ by a factor of 10 or more in detection window.
Metabolism speed — CYP2C9 polymorphisms affect clearance rate. See CYP450 metabolism.
Hydration status — affects concentration per unit volume but not total body burden. Labs correct for this with creatinine normalization.
Exercise timing — intense exercise can temporarily increase plasma THC by mobilizing it from fat. See exercise page.
Dose potency — modern high-potency cannabis produces higher peak and sustained plasma concentrations than 1990s-era product.