Hair testing provides a long-term look at substance use, capturing a history that other tests, like urine or saliva, cannot. When cannabis is consumed orally as edibles, the active compounds are processed by the body and incorporated into the growing hair strand. The test analyzes a small sample of hair cut close to the scalp to detect the presence of the parent drug and its metabolites, which are byproducts created as the body breaks down the substance. While the science of how compounds are trapped in the hair shaft is similar for all drugs, the specific metabolism of edibles affects the final result.
How Substances Enter the Hair Shaft
Hair drug testing relies on the biological process of hair growth. Once a substance is consumed, it enters the bloodstream and circulates throughout the body. The hair follicle is surrounded by blood vessels, and as hair is formed, compounds and their metabolites present in the blood, sweat, and sebum are incorporated into the hair’s inner structure, the keratin matrix.
The hair strand then grows out from the scalp, locking the substance’s chemical signature into its length. Since head hair grows at a consistent rate of about half an inch (1.5 cm) per month, a standard 1.5-inch hair sample reflects a history of use over the preceding 90 days. This mechanism identifies chronic or repeated use rather than a single, acute event.
The Metabolic Difference of Edible THC
The metabolism of Delta-9-tetrahydrocannabinol (THC) changes significantly when consumed orally rather than inhaled. After ingestion, THC is absorbed through the digestive system and travels to the liver, where it undergoes extensive first-pass metabolism. Liver enzymes convert Delta-9 THC into 11-hydroxy-THC (11-OH-THC), a highly psychoactive metabolite.
The 11-OH-THC is then further metabolized into the primary, non-psychoactive compound targeted by drug tests: 11-nor-9-carboxy-THC, or THC-COOH. Both the parent compound (THC) and the final metabolite (THC-COOH) circulate in the bloodstream and are incorporated into the growing hair. Detection of THC-COOH is considered definitive proof of systemic drug use, as this metabolite is created only inside the body and is not found as an environmental contaminant.
Although the route of consumption changes the ratio of parent drug to metabolite in the blood, the hair test focuses on the presence of these compounds above a set threshold.
Detection Windows and Influencing Factors
The standard detection window for a hair drug test is approximately 90 days, based on analyzing a typical 1.5-inch length of hair. This three-month look-back period is established by the hair’s growth rate of about 0.5 inches per month. There is a latency period of about five to seven days after consumption before the hair containing the substance grows out of the scalp and is available for collection.
The most significant factor influencing the detection of edible THC is the frequency and dosage of consumption. Chronic, heavy use leads to higher concentrations of THC and THC-COOH being incorporated into the hair shaft, making detection more likely than a single use. Since individuals have varying hair growth rates, the 90-day window is an average that can be slightly longer or shorter.
Laboratories consider the possibility of environmental contamination and use chemical washing procedures to remove external residues. A positive result requires the detection of the internal metabolite, THC-COOH, above a specific cutoff level. A positive hair test relies on repeated systemic exposure that allows the compounds to deposit consistently into the growing hair shaft over the detection window.