Hair testing and cannabis: what hair really detects

The general recommendations of the Society of Hair Testing, published in 2022, set the methodological benchmarks. The growth rate used is approximately 1.0 cm per month, explicitly presented as approximate and variable between individuals. Sampling is taken at the posterior vertex.

The 2021 consensus devoted to narcotics specifies that the threshold applies to head hair and to segments not exceeding 3 cm, roughly three months subject to that variability.

Two warnings appear there in black and white. The concentration measured does not make it possible to determine the quantity consumed. And it can be affected by external contamination, as well as by dyes, perms and exposure to ultraviolet light.

The 2021 consensus adopts a threshold of 50 pg/mg for THC as for CBD. For THC-COOH, it states not a decision threshold but an analytical performance requirement: a minimum limit of quantification of 0.2 pg/mg.

The reason for this architecture goes back to work by the Institute of Legal Medicine in Strasbourg published in 1995, which already set out the argument: identifying THC-COOH documents consumption far better than detecting the parent molecule, which can come from environmental exposure in a smoky atmosphere.

A study published in 2015 in Scientific Reports pushed the demonstration further, under an unambiguous title: finding cannabinoids in a hair does not prove cannabis consumption. In volunteers who had received THC orally for thirty days, no THC was detected in head hair, beard or body hair. THC-COOH, however, appeared even in segments that had already grown before intake, a sign of transfer through sebum and sweat rather than of incorporation datable through the blood.

A more recent piece of work, published in 2026 in Drug Testing and Analysis, analysed 639 hair samples in the context of driving-licence reinstatement monitoring. Across one hundred samples positive for THC-COOH alone, wash tests were negative, confirming an internal origin. Its authors conclude that detecting THC-COOH, even without THC, constitutes valid evidence of cannabis use. Conversely, 52 samples were positive for THC without the metabolite: the typical profile of contamination.

It splits in two, according to the route of exposure, and the answers diverge.

For a smoked flower, the risk of external deposition is considerable. A study published in 2024 in Drug Testing and Analysis exposed virgin hair strands to smoke in a confined system. A product at 0.4% THC, compliant with the French threshold, deposited 35.54 ng/mg of THC on the hair, that is 35,540 pg/mg, roughly 710 times the positivity threshold of 50 pg/mg. Deposition was comparable when smoking and when vaping.

For a consumed extract, the picture is the reverse. A study published in 2018 in Forensic Science International followed regular consumers of CBD-rich extracts, from 4 to 128 mg of CBD per day. Hair CBD ranged from 10 to 325 pg/mg, but THC was detected in only one sample, below the laboratory's threshold.

A third case is worth knowing. A study published in 2019 in Scientific Reports had a cosmetic hemp oil applied to the hair of ten volunteers for six weeks. The product contained no detectable THC, and yet 89% of the volunteers incorporated at least one cannabinoid, with hair THC reaching 0.13 ng/mg, above the threshold. THC-COOH was detected in none of them.

The honest summary fits in one sentence: yes, perfectly legal use can produce hair THC above the threshold, and no, it does not produce THC-COOH. The laboratory still has to look for the metabolite.

One factor makes matters worse. The report published in December 2023 with the support of MILDECA, covering 248 products bought on the French market, records an average THC content of 0.1%, with 88% of samples below 0.3%, but 30 illicit samples between 0.4 and 1.2%. Counting THC and its acidic precursor together, one sample in five would tip into illicit status.

Not at the roadside. Articles R. 235-1 et seq. of the highway code provide for screening by saliva or urine collection, followed by a confirmation analysis on a blood sample. Hair appears nowhere in them. Our article on driving after consumption details this regime, and those devoted to the saliva test and to drug screening in general cover the common matrices.

Hair analysis in France falls within the field of judicial expert assessment. The consensus of the French Society of Analytical Toxicology recalls the logic of the matrices: urine documents recent consumption, hair documents repeated or chronic consumption. It also requires that the decontamination procedure always be specified, and that an unconfirmed immunoassay result be treated as unacceptable.

We found no official source documenting systematic use of hair testing by French family courts. That observation is the result of a documentary search, and we will not state it as fact. How long cannabinoids remain in the body is covered in our dedicated article on how long CBD stays in the body, and the workplace setting in the one on employers and saliva testing.

Can a legal CBD consumer test positive on a hair test?

For THC, yes, if the exposure is external. A 2024 study shows that a 0.4% THC product smoked in a confined space deposits roughly 710 times the positivity threshold on hair. For THC-COOH, the metabolite that attests to passage through the body, the available studies find no positivity under these conditions.

Why look for THC-COOH rather than THC?

Because THC settles from the outside, through smoke, hands or a cosmetic, whereas THC-COOH is produced by metabolism. A 2026 study of 639 samples confirmed, through negative wash tests, the internal origin of the samples positive for the metabolite alone.

What period does a hair test cover?

The international consensus applies the thresholds to segments of at most 3 cm, for growth of approximately 1.0 cm per month, that is roughly three months. This rate is explicitly presented as variable between individuals, and the concentration measured does not allow the quantity consumed to be deduced.