THC-COOH in Older Hair Segments Complicates Dating Cannabis Exposure
A THC-COOH finding in a particular hair segment does not necessarily date cannabis intake to the month that segment originally grew. A small controlled oral-THC experiment found the metabolite in hair that predated the dosing period.
The dosing history was known
Two male volunteers took 2.5 mg dronabinol three times daily for 30 days. Researchers collected scalp, beard and body hair, as well as combined sebum/sweat specimens. A separate experiment involved one volunteer receiving THCA-A.
Parent THC was not detected in the hair collected during the dronabinol experiment at a detection limit of 1 pg/mg. THC-COOH was detected, including in older scalp-hair segments corresponding to periods before the controlled intake. The metabolite was also measured in sebum/sweat.
Growth position and entry into hair are different
Hair may acquire a substance after the shaft has already formed. The sebum/sweat findings support a contribution from secretions to metabolite distribution along existing hair. THC-COOH was detectable in beard hair for up to eleven weeks after the dosing period, further complicating a simple calendar interpretation.
The researchers also discussed possible transfer through close contact. They did not conduct a large controlled experiment establishing how often such transfer produces positive results in another person. A possible pathway should not be treated as proof that it explains a particular child’s or partner’s specimen.
A small mechanistic study cannot supply a universal rule
Only two people received dronabinol, and the oral dose and concentration profile differ from heavy smoking. The authors acknowledged that measurable bloodstream incorporation at higher exposures could not be excluded. The parent-THC nondetections therefore do not prove that systemic incorporation is impossible in every user.
For a chronology opinion, distinguish the estimated growth interval from the time a compound entered the shaft. Segment length, growth assumptions, sampling site, secretions, cosmetic history and the laboratory method all matter. This experiment challenges precise retrospective dating; it does not turn hair into a measure of impairment or establish an exact alternative date.
Related: interpreting cannabis hair findings and contamination.
Primary research
Finding cannabinoids in hair does not prove cannabis consumption. DOI: 10.1038/srep14906.
