Carboxy-THC in Washed Hair: Still the Reliable Indicator of Marijuana Ingestion

The presence of the metabolite 11-nor-9-carboxy-delta-9-tetrahydrocannabinol (C-THC) in hair is generally accepted as the definitive proof of delta-9-tetrahydrocannabinol (THC) ingestion. During hair analysis, the removal of any potential C-THC external contamination that could result from marijuana smoke or close personal contact via a wash procedure is critical. Here, we performed a series of experiments to demonstrate that C-THC is the reliable indicator of marijuana ingestion when paired with the correct washing procedure to remove potential external contamination.