Cannabis hair test: why THC in hair does not prove use
A hair test can find THC in the hair of someone who has never used cannabis, because smoke, hands or a cosmetic oil can deposit it there. Passage of THC through the body is shown by THC-COOH, a metabolite made by the liver, and only if the laboratory looks for it. For a CBD user, the risk depends on how the product is used.
Key takeaways
- A hair test is positive above a recommended 50 pg of THC per mg of hair, measured on a segment of 3 cm at most, roughly three months of growth.
- THC-COOH is sought at much lower levels: the laboratory must be able to quantify it from 0.2 pg/mg.
- In a lab experiment, smoke from a flower at 0.4% THC left about 35,540 pg/mg of THC on clean hair locks, over 700 times the cut-off.
- Hair THC stayed negative in regular users of CBD-rich extracts.
- French roadside drug tests rely on saliva and never on hair.
How a hair analysis is read
Hair records what circulated in the blood while it was growing. The general recommendations of the Society of Hair Testing (SoHT), an international scientific society, take a growth rate of about 1.0 cm a month while stressing that it differs between individuals. The lock is cut at the back of the crown, where growth is most regular.
Interpretation rules come from the SoHT consensus on drugs of abuse, finalised in 2021. The cut-off applies to head hair and to segments of 3 cm or less, since a longer segment dilutes the result. The text sets three limits as well. The measured concentration cannot be used to calculate the amount consumed. Dyeing, bleaching and UV exposure can alter it. And analysing the wash water helps detect external contamination.
The French Society of Analytical Toxicology had already summarised the logic of the different samples in its 1999 consensus: urine reflects recent use, hair repeated or chronic use. The same text calls "unacceptable" any result that rests on an immunoassay screen alone, without confirmation by a reference method.
The cut-offs, and why the metabolite decides
The 2021 consensus sets 50 pg/mg for THC and 50 pg/mg for CBD; a picogram per milligram is one billionth of a gram of molecule per gram of hair. For THC-COOH the text gives no decision cut-off. It sets a sensitivity requirement instead: the laboratory must be able to quantify it from 0.2 pg/mg. Detecting it, the consensus states, strongly supports THC use.
The argument dates back to work by the Institute of Forensic Medicine in Strasbourg, published in 1995 in the Journal of Forensic Sciences. The authors explained that THC-COOH documents use more reliably than THC, which can come from a smoky atmosphere. Of the 30 samples analysed, 17 contained the metabolite, at 0.02 to 0.39 ng/mg, or 20 to 390 pg/mg.
A University of Freiburg study published in 2015 in Scientific Reports states its finding in the title: finding cannabinoids in hair does not prove cannabis consumption. Volunteers who took THC by mouth showed no relevant amount of it in their hair. THC-COOH, by contrast, also appeared in segments that had grown before the dose, and in sweat and sebum. According to the authors, all three molecules can pass from a user to someone close by through hands, sweat or smoke, and child-protection cases are among the situations they name.
A Belgian study published in 2026 in Drug Testing and Analysis brings field data from 639 hair samples from drivers monitored to regain their licence. Of these, 76 were positive for THC-COOH alone and 52 for THC alone. Wash tests on the samples positive only for the metabolite came back negative, which points to an internal origin. The authors conclude that THC-COOH is valid evidence of use even without THC, while samples positive for THC without the metabolite match a contamination profile.
CBD users: three very different situations
How the product is used makes all the difference, and the available studies quantify it.
Flowers that are smoked or vaped deposit THC. A team in Bologna exposed clean hair locks to cannabis smoke and vapour in a small closed system. In their study, published in 2024 in Drug Testing and Analysis, a product at 0.4% THC, just above the French limit of 0.30%, deposited an average of 35.54 ng/mg of THC, or 35,540 pg/mg. Cannabis at 9.7% deposited 169.30 ng/mg. Smoking and vaping did not differ significantly. It was an in vitro experiment in a confined space, with no measurement on users.
Extracts taken by mouth leave little THC in hair. A Luxembourg study published in 2018 in Forensic Science International followed regular users of CBD-rich extracts, who reported taking 4 to 128 mg of CBD a day, or 0.1 to 4.3 mg of THC. CBD in their hair ranged from 10 to 325 pg/mg, while THC turned up in a single sample, below the laboratory's cut-off.
Hemp oil applied to the hair can deposit THC. In a study published in 2019 in Scientific Reports, ten volunteers put a commercial hemp oil on their hair every day for six weeks. An analysis of the oil had found no THC. Even so, 89% of the volunteers then had at least one cannabinoid in their hair, with THC up to 0.13 ng/mg, or 130 pg/mg. None had THC-COOH. These products are covered in our article on CBD for hair and scalp.
In two of the three cases, outside exposure can take hair THC above the cut-off with no THC-COOH at all. The outcome then turns on what the laboratory searches for.
The product itself is a final factor. The MILDECA-supported report of December 2023 analysed 248 products bought in France, and 30 of them exceeded the limit, at 0.4 to 1.2% THC. Once THCA, the acid form that heat converts into THC, was counted, 21% of samples went above 0.3%. Using such a product brings THC into the body, and with it THC-COOH. How that level is measured is explained in our guide to the legal THC level.
Where hair tests are used in France
Roadside testing does not use them. Drug testing of drivers, governed by articles R235-1 onwards of the French Highway Code, relies on a saliva test. Confirmation comes from a saliva sample analysed in a laboratory, or from a blood sample if the driver requests a second opinion. That chapter never mentions hair. Penalties are covered in our article on CBD and driving, and what the saliva test measures in our saliva test guide.
Hair analysis is used in court-ordered expert reports, abstinence monitoring and child-protection cases. The Belgian study cited above concerned drivers regaining their licence in Belgium. For workplace testing, see our article on employers and saliva tests.
If you face a hair result, ask the laboratory three questions. Was THC-COOH tested for, and at what limit of quantification? Was the wash water analysed? Was the tested segment 3 cm or shorter?
Frequently asked questions
For THC, yes, particularly when flowers are smoked or vaped or hemp oil is applied to the hair. For THC-COOH, the metabolite showing that THC went through the body, the studies cited here found no positive result from outside exposure alone. The picture changes with a product whose THC exceeds the legal limit.
THC can reach hair from outside, through smoke, hands or a cosmetic, whereas THC-COOH is produced by the liver. In the 2026 Belgian study, wash tests on samples positive for the metabolite alone were negative. The Freiburg study notes, however, that a user's sweat can transfer THC-COOH to someone close.
The cut-off covers a segment of 3 cm or less. With growth of about 1.0 cm a month, that is roughly three months. Growth differs between people, and the concentration does not tell you how much was consumed.
According to the SoHT consensus, bleaching, dyeing, straightening or UV can alter the measured concentration, so these treatments should be noted when the sample is taken. Our article on CBD in the body covers how long cannabinoids remain in the body.
Written by the Phytogrammes team
Each journal article draws on cited primary sources: ANSM, EFSA, EUR-Lex and peer-reviewed publications. Product profiles are distinguished from documented laboratory results. Our approach.
Article published on . CBD is not a medicine.