Hair Test for Marijuana Proves Nothing

This article was originally posted on RealClearScience.

There are few things that pot enthusiasts dread more than the unannounced drug test. A positive test for THC, the active ingredient in marijuana, can result in the loss of a job or child custody. But new research by a team of German scientists suggests that detecting THC in a hair sample doesn’t prove cannabis consumption.

To understand why, it is first necessary to explain the basic chemistry of THC metabolism. (See bottom part of figure.)

The molecule found in fresh marijuana, THCA-A, easily converts to the psychoactive THC upon heating. (That is why marijuana is smoked or baked into brownies.) After consuming it, the body converts THC into 11-nor-9-carboxy-THC (a.k.a. THC-COOH). Presence of this molecule in hair is considered definitive proof that a person consumed pot. However, due to its low concentration, it is difficult to detect. As a result, most hair tests only look for the presence of THC. However, according to the latest study, this is very problematic.

Two volunteers who had neither used nor were anywhere near marijuana prior to the study consumed 2.5 mg THC three times per day for 30 days. Despite having detectable levels of THC in their blood, the team found no trace of THC in their hair. This suggests that the only way for THC to get into a person’s hair is through the environment. Thus, a negative test for THC proves absolutely nothing (as a person could eat a pot brownie and still produce a negative test result). Far more disturbingly, a nonsmoker who spends time with pot smokers may produce a positive result on a THC hair test.

The metabolite 11-nor-9-carboxy-THC (THC-COOH) was found in both blood and hair samples from the volunteers. However, the hair samples contained THC-COOH from the time period during which the volunteers were ingesting THC, as well as before the time they were ingesting it. Because THC-COOH is secreted by oil glands, it is likely that the metabolite contaminated the entire hair shaft. From a forensic standpoint, this is not good, as it suggests that THC-COOH can be transferred from one person to another via oil secretions. Indeed, THC-COOH has been detected in the hair of children younger than two years old. Obviously, these youngsters aren’t smoking the ganja quite yet. Contamination from family members is almost certainly the correct explanation.

Obviously, this experiment needs to be repeated with a much larger sample size. But, its results cast deep suspicion on the reliability of hair tests for marijuana.

Source: Bjoern Moosmann, Nadine Roth & Volker Auwarter. “Finding cannabinoids in hair does not prove cannabis consumption.” Scientific Reports 5, Article number: 14906. Published online: 07-October-2015. doi:10.1038/srep14906