How THC Acts in the Body: Duration, Metabolism, and Detection

How long tetrahydrocannabinol (THC) affects you depends mainly on the dose, route of administration, product formulation, frequency of use, and individual differences in metabolism. The noticeable effects may last several hours, but THC and its metabolites can remain in the body much longer.

Pharmacokinetics and pharmacodynamics

Pharmacokinetics describes what the body does to a substance: how it is absorbed, distributed, metabolized, and eliminated. Pharmacodynamics describes what the substance does to the body, including how it interacts with cannabinoid receptors and produces effects such as euphoria, sedation, increased appetite, impaired coordination, or changes in perception.

These processes are related but not identical. THC levels in the blood can fall rapidly after inhalation even while some effects continue, and the presence of THC or its metabolites in a test sample does not by itself establish current impairment.

How long do the effects of THC last?

Route of administration makes a major difference:

• Smoking or vaporizing: Effects can begin within minutes. Blood concentrations rise quickly and then decline rapidly, while the acute intoxicating effects commonly last a few hours. The amount absorbed varies with the product, inhalation technique, and dose.
• Edibles and other oral products: Effects usually begin more slowly—often after 30 to 90 minutes—and may not peak for one to several hours. Because the dose passes through the digestive system and liver, oral THC can produce a longer-lasting and less predictable experience than inhaled cannabis.

In a controlled human study of cannabis brownies containing 10, 25, or 50 milligrams of THC, subjective and cognitive effects generally peaked between 1.5 and 3 hours and lasted about 6 to 8 hours. Actual experiences can be shorter or longer, particularly with high doses, repeated dosing, or products with uncertain THC content. The controlled oral-cannabis pharmacokinetic study also found that THC was detectable in whole blood for up to about 22 hours under the study’s testing conditions.

THCA and the formation of THC

In growing cannabis plants, much of the cannabinoid is present as tetrahydrocannabinolic acid, or THCA, rather than neutral THC. Heating causes THCA to lose a carboxyl group in a reaction called decarboxylation, forming THC. This occurs during smoking, vaporizing, and the preparation of many edible products.

Unheated THCA does not produce the same intoxicating effects as THC. Laboratory and preclinical research has investigated possible anti-inflammatory, neuroprotective, and other properties of THCA, but those findings should not be treated as proof of established clinical benefits in humans. A scientific review of THCA and its decarboxylation to THC summarizes the evidence and its limitations.

How the body metabolizes THC

THC is highly lipid-soluble, meaning it readily distributes into fatty tissues and other organs. It is extensively metabolized, primarily in the liver, by cytochrome P450 enzymes, including CYP2C9, CYP2C19, and CYP3A4.

The main pathway is:

1. THC is converted into 11-hydroxy-THC (11-OH-THC), an active metabolite with psychoactive effects.
2. 11-OH-THC is further oxidized into 11-nor-9-carboxy-THC, commonly called THC-COOH. This metabolite is not considered intoxicating and is the principal target of many cannabis urine tests.
3. THC-COOH and other metabolites undergo phase II reactions, including glucuronidation, which makes them easier for the body to eliminate.

Most THC metabolites leave the body through feces, with a smaller portion excreted in urine. The specific metabolites found, and their concentrations, depend partly on whether THC was inhaled or taken orally.

Oral THC undergoes substantial “first-pass” metabolism in the liver before reaching the general circulation. This helps explain why edibles have a delayed onset and why they can produce relatively more 11-OH-THC than inhaled cannabis. The National Cancer Institute’s overview of cannabis pharmacology summarizes the differences between oral and inhaled THC.

Half-life is not the same as duration of the high

A drug’s half-life is the time required for the amount or concentration being measured to decrease by half. It does not indicate exactly how long a person will feel intoxicated, nor does it provide a reliable prediction of how long a drug test will remain positive.

THC has a rapid initial decline in the blood after inhalation as it moves from the bloodstream into tissues. A slower terminal phase can follow as THC is gradually released from lipid stores and metabolized. Repeated use can increase tissue accumulation and prolong the elimination phase.

The often-cited figures of approximately 1.3 days for infrequent users and 5 to 13 days for frequent users refer primarily to the urinary excretion of THC-COOH—not to the duration of THC’s intoxicating effects. In the original study of urinary THC-COOH excretion in frequent and infrequent users, the early mean excretion half-life was 1.3 days in infrequent users and about 1.4 days in frequent users, while some frequent users showed much longer terminal half-lives of up to roughly 10 days.

Why detection can outlast impairment

THC and its metabolites can persist after the noticeable effects have ended. Detection windows vary with frequency of use, dose, body composition, route of administration, the biological sample being tested, the laboratory method, and the test’s cutoff concentration.

Urine tests generally look for THC-COOH or its conjugates rather than active THC. A positive urine result therefore indicates prior exposure, not necessarily recent use or current impairment. In frequent users, residual cannabinoids may remain detectable for weeks, and in some cases longer, because of gradual release from tissue stores.

Genetic variation in drug-metabolizing enzymes, other medications, liver function, and product characteristics can also affect THC exposure. For that reason, there is no single timetable that accurately predicts when every person will feel no effects or test negative.

The practical takeaway

Inhaled THC acts quickly and usually has a shorter acute duration, while oral THC takes longer to begin, may feel stronger or less predictable, and can last substantially longer. THC is converted first to active 11-OH-THC and then largely to inactive THC-COOH, which is eventually modified and excreted.

Understanding this pathway helps explain why the subjective “high,” blood concentrations, and drug-test results follow different timelines. It also underscores why a test result should not be used on its own to determine whether someone is currently impaired.

For a detailed clinical discussion of absorption, distribution, metabolism, excretion, receptor effects, and drug interactions, Health Canada’s guidance for health professionals on cannabis pharmacology provides a comprehensive reference.