Hair Follicle Test
A drug test that detects cannabis metabolites trapped in growing hair, offering the longest detection window of common test types.
Hair tests are the boogeyman of weed drug screening — they look back about 90 days, not the 1–30 of urine or saliva. The good news: they're uncommon outside of certain employers (federal contractors, some safety-sensitive jobs) and child-custody cases. The bad news: shampoos and home detox kits have weak evidence behind them, and external smoke exposure can cause positives, which is exactly why labs require confirmatory testing before calling a result.
What it is
A hair follicle test is a laboratory drug screen that analyzes a small hair sample — typically about 1.5 inches cut close to the scalp — for drug metabolites incorporated into the hair shaft as it grows. Because head hair grows roughly 0.5 inch per month, a standard sample reflects approximately the past 90 days of exposure [1] Strong evidence.
For cannabis, labs look for THC-COOH (11-nor-9-carboxy-THC), the same non-psychoactive metabolite targeted in urine tests. It enters hair via blood supply to the follicle and, to a lesser extent, via sweat and sebum on the scalp surface [2].
How it works
Testing is a two-step process. Initial screening uses ELISA immunoassay to flag presumptive positives. Any positive is then confirmed using GC-MS/MS or LC-MS/MS, which can distinguish active metabolites (proof of ingestion) from parent THC alone (which could indicate external smoke contamination) [3] Strong evidence.
The U.S. Substance Abuse and Mental Health Services Administration (SAMHSA) has proposed federal hair testing guidelines with a confirmatory cutoff of 1 pg/mg for THC-COOH [1]. Many private labs use similar or slightly different thresholds.
What it does — and doesn't — do
Does:
- Detects chronic or repeated cannabis use over roughly 90 days [1] Strong evidence.
- Provides a longer detection window than Urine Drug Test or Saliva Drug Test.
Doesn't:
- Reliably detect single or very infrequent use. Light users often test negative [2] Strong evidence.
- Tell you when in the 90-day window use occurred with precision.
- Easily distinguish use from heavy passive smoke exposure without confirmatory testing — a known issue that's driven both lab protocol changes and litigation [4] Strong evidence.
Folklore vs. evidence: Macujo, Jerry G, and similar bleach/vinegar shampoo protocols are widely sold online. Controlled research on their effectiveness is essentially nonexistent Anecdote. Body hair can be used when head hair is unavailable, but it grows more slowly and reflects a longer, less defined window [1].
Sources
- Government Substance Abuse and Mental Health Services Administration (SAMHSA). Proposed Mandatory Guidelines for Federal Workplace Drug Testing Programs Using Hair. Federal Register, 2020. ↗
- Peer-reviewed Huestis MA, Gustafson RA, Moolchan ET, et al. Cannabinoid concentrations in hair from documented cannabis users. Forensic Science International, 2007;169(2-3):129-136.
- Peer-reviewed Cooper GAA, Kronstrand R, Kintz P. Society of Hair Testing guidelines for drug testing in hair. Forensic Science International, 2012;218(1-3):20-24.
- Peer-reviewed Moosmann B, Roth N, Auwärter V. Hair analysis for THCA-A, THC and CBN after passive in vivo exposure to marijuana smoke. Drug Testing and Analysis, 2014;6(1-2):119-125.
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