The results strongly suggest that THCA-A is not incorporated into hair through the bloodstream or via sebum/sweat to a relevant extent. Although this was tested only in one individual, the daily dose of THCA-A was at least an order of magnitude higher than expected in excessive cannabis smokers. Therefore, the THCA-A detected in forensic hair samples (concentration range in hair samples of cannabis consumers: 46–4,700 pg/mg13) can only be explained by external contamination via handling of cannabis material15.

The incorporation rate of THC via the bloodstream into the hair seems to be negligible low as no THC could be detected in the hair samples of the participants after systemic dronabinol uptake. It follows from Fick’s law that the amount of analyte incorporated into hair should be proportional to the area under the analyte serum concentration over time curve (AUC). Given that the THC AUC 0→24 h of the two participants was only less than five times lower than the AUC range found in the literature for occasional cannabis smokers after a single consumption (780–6,300 μg/L * min)19,20, it is obvious that also no relevant incorporation through the bloodstream into hair is expected to occur in cannabis users and THC detected in forensic hair samples does originate from external sources. To reach THC concentrations of 50 pg/mg (cut-off recommended by the Society of Hair Testing21) through incorporation via the bloodstream would require consumption of extremely high amounts of THC, which would certainly be associated with a several-fold higher amount of THC incorporated through contamination routes (cannabis smoke exposition and/or transfer by contaminated fingers). Consequently, THC findings in hair cannot be regarded as a proof of cannabis consumption. At the same time, oral uptake of THC or cannabis products does not necessarily lead to positive THC hair findings, which can be of interest in abstinence control.

Furthermore, the detection of THC-COOH in hair segments did not correlate to the period of THC intake and the presence of THC-COOH in sebum/sweat implicates a relevant contribution to the THC-COOH findings in hair samples by diffusion of the analyte from sebum into the hair matrix. The marked variations in the THC-COOH concentrations between body regions may be explained by differences in the physiology (e.g. presence of apocrine sweat glands in the axillary and pubic region), sampling particularities (e.g. regular shaving of beard hair vs. sampling of hair strands) and a possible transfer of the analyte due to contamination of hair with urine (pubic region). The fact, that THC-COOH was detectable up to 11 weeks past the intake period in beard hair further underlines a relevant incorporation via secretion of sebum which shows a physiological time shift, or by diffusion from surrounding tissues2.

At first glance, differentiation of the route of THC-COOH incorporation into hair seems irrelevant as long as positive THC-COOH findings in hair require THC uptake by the individual under investigation. However, considering the presence of THC-COOH in sebum/sweat, a transfer to other persons’ hair is possible. This is particularly true for young children or partners of cannabis consumers (close body contact, sleeping on the same pillow etc.). Comparing the maximum serum THC-COOH concentrations detected in persons massively exposed to cannabis smoke in a ‘coffee shop’ (0.5–1.7 ng/mL)22 to the maximum serum concentrations determined in our study (18 and 40 ng/mL), it seems very unlikely that passive smoke exposition can lead to similar THC-COOH concentrations in hair as chronic active consumption does. However, THC-COOH can be detected in hair of young children (age: <2 years)23 in concentrations similar to the concentrations detected in the hair after oral dronabinol intake. Therefore, it seems much more plausible that THC-COOH is transferred to the children’s hair by close contact to the cannabis consumers in the family context rather than by systemic uptake after exposition to cannabis smoke.

Limitations of the study

For the oral intake of THCA-A only one individual was tested. Although the extraordinary high serum concentrations reached should compensate for this, physiological characteristics of the individual may have led to THCA-A not being incorporated into hair to a measurable extent. In the study with oral intake of dronabinol a relatively low dose of THC was used, which may reflect THC uptake of moderate cannabis smokers, but not of heavy users. Therefore, measurable incorporation of THC from the blood stream cannot be excluded in the case of heavy cannabis smoking. Due to oral administration (slow resorption, first-pass effect) the maximum THC serum concentrations were lower than the maximum concentrations generally reached after smoking. Although – following from Fick’s law – incorporation should be proportional to the AUC, the diffusion coefficient may vary with the gradient. High concentration gradients as observed directly after smoking might therefore lead to a more efficient incorporation of THC. Furthermore, the number of individuals tested in this study was low (n = 2) and pharmacokinetic particularities may affect the generalizability of the findings.