How Long Is Weed in Your Hair? Understand THC Detection Times

How long is weed in your hair detectable by a drug test?

Weed, or THC, can typically be detected in hair samples for up to 90 days after use. Hair testing identifies THC metabolites that accumulate in the hair shaft as it grows. Since head hair grows about half an inch per month, a standard 1.5 inch sample reflects roughly three months of history.

This detection window is significantly longer than urine or blood testing, which usually detect THC for only days to weeks. Hair testing is preferred for long-term usage verification or legal and employment purposes.

What is the process for THC hair testing at AMH Nationwide?

AMH Nationwide coordinates the lab order and directs clients to one of over 20,000 nationwide collection sites affiliated with five major national lab networks: Quest Diagnostics, LabCorp, Omega Labs, United States Drug Testing Labs, and ExperTox. There is no online ordering; all test requests require contacting AMH directly.

At collection, a small hair sample is taken from the head or body. The sample is sent to the lab where it undergoes screening and confirmation testing. Every non-negative result is reviewed by a certified Medical Review Officer (MRO) for accuracy and interpretation.

How does THC get into hair and what affects detection?

THC enters hair primarily through the bloodstream after consumption. As hair follicles grow, THC metabolites are deposited inside the hair shaft. External contamination from smoke does not generally cause a positive result because labs wash samples before analysis.

Factors influencing detection include hair color, thickness, and cosmetic treatments. Darker hair may bind more THC, while bleaching or dyeing can reduce detectable levels. Frequency and amount of use also impact metabolite concentration.

Does one-time or infrequent use show up in hair testing?

One-time or infrequent use of weed can sometimes be detected, but low metabolite levels may fall below test cutoffs. Hair tests are more reliable at detecting repeated or chronic use due to accumulation of THC metabolites over time.

Because hair testing covers a longer time frame, a single use might be missed if the sample length corresponds to a period before consumption or if the amount used was very small.

Is hair testing for THC accepted by DOT and other employers?

For DOT-regulated safety-sensitive positions, urine testing following the federally certified 49 CFR Part 40 urine protocol is mandatory. Hair testing is not permitted for DOT drug tests but is used by many non-DOT employers and legal entities for longer detection windows.

AMH Nationwide offers both urine and hair testing routed through SAMHSA-certified labs. Hair testing is a preferred alternative for non-DOT roles requiring extended detection of past THC use.

How reliable are THC hair test results from AMH Nationwide?

AMH Nationwide uses accredited labs with strict quality controls, including SAMHSA alignment and CLIA certification. All positive or non-negative hair test results undergo review by certified Medical Review Officers who verify validity and rule out potential errors or medical explanations.

This process ensures high reliability and defensibility of THC hair test results for employers, courts, and individuals.

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