One of the drugs screened for routinely in workplace drug testing is marijuana.One of the safe.ways to analyse marijuana is to test for Carboxy THC in hair.Carboxy THC, (THC-COOH), is a metabolite of marijuana that is not normally found in the environment or in smoke and, instead, is made by the body in processing the drug.Parent THC, on the other hand, may be found on the hair as a result of external contamination.