Hair follicle testing is a specialized method used to screen for drug use over an extended timeframe. This process differs significantly from blood or urine analysis, which typically only reveal substance use within a few hours to a few days. The hair test is designed to detect the presence of drug compounds that have become physically incorporated into the growing hair strand.
How Cannabis Metabolites Enter the Hair
When cannabis is consumed, the primary psychoactive compound, delta-9-tetrahydrocannabinol (THC), is processed by the body. This metabolism produces various byproducts, most notably 11-nor-9-carboxy-THC (THC-COOH). This metabolite is non-psychoactive and is the compound scientists primarily look for in a hair test to confirm systemic use.
These metabolites circulate throughout the bloodstream, which also nourishes the hair follicle matrix at the base of the scalp. As the hair is formed, the drug compounds and their metabolites are transferred from the blood supply into the core structure of the hair shaft. Once trapped within the keratinized protein of the hair, these compounds remain embedded as the hair grows outward.
The detection of THC-COOH is considered definitive proof of consumption, as this metabolite is only created inside the body. This mechanism makes it challenging to argue that a positive result came solely from external contamination, such as secondhand smoke.
The Standard Detection Period
The core utility of hair testing lies in its ability to provide a look back at an individual’s history of substance use. The standard detection window for a hair test is approximately 90 days, or three months.
Head hair grows at a relatively consistent rate of about 0.5 inches (or 1.3 centimeters) per month. To establish the 90-day window, laboratories typically collect a hair sample that is 1.5 inches (about 3.9 centimeters) long, measured from the root closest to the scalp. The 1.5-inch segment therefore represents the three most recent months of growth.
Because hair does not begin to show drug use immediately, there is usually a “washout” period of about 7 to 10 days at the beginning of the tested segment. This is the time it takes for the drug-containing hair to emerge from the scalp to be included in the sample. For this reason, hair testing is not effective for detecting very recent use, which is better assessed by urine or blood tests.
Variables Affecting Test Outcomes
While the 90-day window is the standard, several biological and external factors can alter the test’s outcome by influencing metabolite concentration in the hair. One factor is the frequency and dosage of cannabis use. Heavy, chronic users incorporate a much higher concentration of THC-COOH than infrequent users, making detection more likely and extending the effective window.
Hair color also plays an unexpected role in detection due to melanin binding. Drug metabolites, including those from cannabis, tend to bind more readily to melanin, the pigment responsible for darker hair colors. Consequently, individuals with darker hair may show higher concentrations of metabolites compared to those with lighter hair, even with similar consumption levels.
Chemical treatments such as bleaching, dyeing, perming, or relaxing can chemically damage the hair shaft. This damage can cause a reduction in the detectable level of drug metabolites, as the chemicals can partially leach the compounds out of the hair’s structure. While this may reduce the concentration, it is not a reliable method to eliminate evidence of use.
If a scalp hair sample is unavailable because the hair is too short, body hair may be collected instead. Body hair—such as chest, leg, or armpit hair—grows at a much slower and less predictable rate than head hair, and can represent a significantly longer detection window, sometimes up to a year, which makes the timeline of use much harder to pinpoint accurately.
Interpreting Positive Hair Test Results
A positive hair test result confirms that cannabis metabolites were present in the body during the period represented by the tested hair segment. It serves as an indicator of past, systemic use within the approximate 90-day detection window.
It is important to understand what a positive result does not reveal. Unlike blood or breathalyzer tests, a positive hair test cannot determine the precise date of use or the current level of impairment. The concentration of metabolites in the hair does not correlate directly to whether a person was intoxicated at the time of sample collection.
Laboratories employ strict testing thresholds, often based on federal guidelines, to ensure accuracy and minimize the risk of false positives. These cutoff levels are set to distinguish between active consumption and the minimal chance of contamination from environmental exposure.