Hair follicle drug tests typically detect drug use up to 90 days in the past. This window is based on analyzing about 1.5 inches of hair closest to the scalp, which grows at roughly half an inch per month.
Standard Detection Window
Labs standardize tests by collecting 1.5 inches of hair, covering approximately three months of history. Drugs enter the hair shaft 7-10 days after use via the bloodstream, so very recent use may not show up.
Factors Affecting Range
Hair length determines the maximum lookback; longer samples can reveal use over 12 months or more, though rare in practice. Growth rate, drug frequency, and type also influence results, with heavier use potentially extending detectability.
Comparison to Other Tests
Unlike urine tests (days to a week), hair tests excel at long-term patterns but miss acute impairment.