What is a hair follicle drug test?

A hair follicle drug test is a laboratory screening method that analyzes hair samples for evidence of drug or alcohol exposure over an extended period of time. Despite the common name, the test usually evaluates the hair shaft rather than the follicle itself. Drugs and metabolites become incorporated into growing hair and can remain detectable for weeks or months depending on hair length and testing procedures.

Hair testing identifies substances that enter the bloodstream and become embedded within developing hair. As hair grows outward from the scalp, these compounds remain trapped within the hair structure and create a historical record of exposure. Laboratories typically collect scalp hair close to the skin because it provides the most consistent growth timeline.

This testing method is commonly associated with identifying repeated or long-term substance use rather than recent intoxication. Most standard hair testing evaluates approximately 90 days of exposure using about 1.5 inches of scalp hair. Very recent use may not yet appear because hair requires time to grow above the scalp surface.

Hair testing may be used in workplace screening, legal proceedings, monitoring programs, clinical settings, and forensic investigations. Different panels can screen for cannabinoids, cocaine, opioids, amphetamines, benzodiazepines, alcohol metabolites, and other substances depending on laboratory protocols. Confirmation testing is commonly used to improve accuracy after initial screening results.

Laboratories use washing procedures and analytical methods designed to reduce the impact of environmental contamination and improve specificity. Factors such as hair color, cosmetic treatment, growth rate, and substance type may still influence results. Interpretation therefore generally considers both laboratory findings and the broader clinical or forensic context in which testing occurs.