THC detox: why nothing speeds up clearing it for a test

THC, the psychoactive molecule in cannabis, is highly fat-soluble. After inhalation or ingestion, it enters the blood and then moves into fat-rich organs. A reference review published in 2007 sums up the mechanism: with prolonged use, THC concentrates in human fat and stays there for a long time, and this slow release is what lengthens its half-life. In chronic users, the terminal half-life of THC in plasma exceeds 4.1 days.

The liver then converts THC, mainly through the enzymes CYP2C9 and CYP3A4, into 11-OH-THC and then THC-COOH. That last one, which is inactive, is the molecule most urine tests look for. Within five days, 80 to 90% of a dose is eliminated: more than 65% in faeces, about 20% in urine.

This route explains why a "detox" cannot empty the store quickly. THC is not sitting in urine that could be flushed out: it is in fat, and it leaves at its own pace.

The duration depends on the sample, the dose and above all how often someone uses. The figures below come from public sources and controlled studies; they are orders of magnitude, not individual guarantees.

Saliva and blood reflect recent use. According to the detection-time table of Drogues Info Service, the French public drug helpline, THC stays in saliva for 6 to 8 hours after occasional use, and up to 24 hours after regular use. In blood, it can be measured for 2 to 8 hours after occasional use. In heavy users, the fat store changes everything: in a study of 30 daily smokers, 2 of the 5 participants followed through 30 days of abstinence still had THC in their blood, at 0.3 µg/l, at the end of follow-up.

Urine keeps the trace longer, because the test looks for THC-COOH. After a single cigarette, THC-COOH stayed detectable for an average of 34 hours at a low dose and 89 hours at a higher dose, at a 15 ng/ml cutoff. Among 86 chronic users followed in 1985, some were positive for up to 46 consecutive days, and the average elimination time was 27 days.

Hair, finally, grows about 1 cm a month. But it is poor at detecting light use: only 39% of the occasional users in a 2017 study tested positive. Our article on hair testing details its limits.

Drinking lots of water

Water does not touch the THC stored in fat. It dilutes urine, which lowers the THC-COOH concentration for a few hours, and labs know it. A sample is classed as "dilute" when its creatinine is below 20 mg/dl and its specific gravity below 1.003. Among 21,696 samples tested in Taiwan, dilution accounted for 89 to 95% of non-compliant samples. The lab then reports the sample as dilute instead of calling it negative.

Exercising just before

This is the riskiest idea. In a 2013 trial, 14 regular users cycled for 35 minutes: their blood THC rose slightly after the effort, more so in those with a higher body mass index. By burning fat, exercise can release some of the stored THC back into circulation. Right before a blood test, that is the opposite of what you want.

Detox kits, capsules and drinks

None of these products has been shown to speed up THC elimination, and no health authority recognises such an effect. CBD does not mask THC either: our guide to CBD and drug testing cites a study where adding CBD did not lower THC levels.

In France, roadside testing uses saliva. The order of 13 December 2016 sets the detection thresholds: 15 ng/ml of saliva for the roadside test, then 1 ng/ml of saliva or 0.5 ng/ml of blood for the confirmation analysis in a lab. These are technical thresholds, not a tolerance.

Article L235-1 of the Highway Code punishes driving after drug use with 3 years in prison and a €9,000 fine, with no threshold. The Cour de cassation ruled in a judgment of 21 June 2023 that the offence is made out whatever the dose absorbed, even after a legally sold product. The only precaution that works is not driving while THC may be present.

Tests do not look for CBD. But a full-spectrum CBD product legally contains up to 0.30% Δ⁹-THC, and that THC follows the same route as THC from cannabis. Taken every day, it can be enough to trigger a positive test: the drug-testing guide mentioned above describes an oil with 0.02% THC that turned 7 of 14 participants positive in urine within four weeks.

If you have to take a test, only a product whose certificate of analysis shows Δ⁹-THC as not detected really limits the risk. Our guide to reading a certificate of analysis shows where to find that line, and the article on how long CBD stays in the body completes these figures.