Hair follicle drug tests measure metabolites of drugs that are incorporated into the hair shaft as it grows. The detection window for hair follicle drug tests typically ranges from 90 to 180 days, depending on the length of the hair sample collected.
Hair follicle drug tests are often used to detect drug use over a longer period of time than urine or blood tests. They are also less susceptible to adulteration and can be used to confirm drug use in cases where other types of drug tests are negative.