A hair drug test can detect cannabis for up to 90 days, or about three months. Most labs collect a sample about 1.5 inches long from the base of the scalp, and that length covers the last 90 days of hair growth. Longer samples reach further back, with some labs reporting a window of up to six months.
How Often Should You Smoke for a Hair Test to Fail?
Why Hair Holds a Record of Cannabis Use
Hair grows about half an inch per month. As each strand forms inside the follicle, it pulls in compounds circulating in the blood, including THC metabolites. Those metabolites stay trapped inside the strand, so the record of use moves outward as the hair grows.
That growth rate sets the detection window. A 1.5-inch sample covers 90 days. A 3-inch sample covers about six months.
Hair vs. Urine, Blood, and Saliva Tests
Hair keeps a far longer record than any other sample type, which is why employers use it for pre-employment screening.
- Hair: up to 90 days, longer with a longer sample
- Urine: 1 to 30 days
- Blood: 1 to 2 days, up to 7 days with heavy use
- Saliva: 1 to 3 days
Hair also has a blind spot at the front end. It cannot detect use from the last 7 to 10 days, because those strands have not grown out of the follicle yet.
What Determines How Long THC Stays in Your Hair?
Four things shape the result: how much you use, how long your hair is, what you do to it, and which hair the lab collects.
How Often You Use
One session may leave too little THC-COOH to reach the cutoff. Daily or near-daily use builds higher levels in the strand and raises the odds of a positive result.
Hair Length and Growth Rate
Labs test the first 1.5 inches unless a longer history is requested. A fast growth rate means a longer sample covers more time, while slow growth packs the same period into less length.
Bleaching, Dyeing, and Relaxing
Chemical treatments break down some of the metabolite inside the strand. Treated hair can test lower, but results vary, and severe damage may cause a lab to question the sample.
Scalp Hair vs. Body Hair
Body hair grows slower than scalp hair, so a body hair sample can stretch the window past 90 days. Labs fall back to body hair when a donor has little or no scalp hair.
Secondhand Smoke
Passive exposure can leave THC on the outside of the shaft. Most labs wash the sample with solvent before analysis, and the 1 pg/mg cutoff screens out incidental contact. A positive result from secondhand smoke is unlikely.
What Do Hair Tests Actually Detect?
Hair panels look for THC-COOH, the main metabolite of delta-9 THC. The parent compound is not the target. A common cutoff is 1 picogram per milligram (pg/mg), though some labs run lower thresholds near 0.1 pg/mg.
An immunoassay handles the first screen. Any sample above the cutoff goes to mass spectrometry (GC-MS or LC-MS) for confirmation, which is why a hair result holds up in workplace appeals and legal challenges.
What Happens During Collection?
A collector cuts a lock of hair about the width of a pencil from the crown of the head, as close to the scalp as possible. The sample is sealed and sent to a lab, where it is washed, cut into segments, and tested. Results reach the employer in 1 to 3 business days after the lab receives the sample.
Can You Pass a Hair Test?
No shampoo, mask, or detox kit has been shown to clear a hair test in published research. Products that make that claim can damage the hair or leave residue that draws a lab's attention. Time is the only factor that works: wait out the 90-day window, or keep use infrequent enough to stay under the cutoff.
How Far Back Can a Hair Test Reach?
Most panels ask for a 90-day history, which matches the standard 1.5-inch sample. Some labs analyze a longer sample or segment the hair month by month, which can push the lookback to six months or more. Very old segments give weaker readings because of sunlight, washing, and normal wear.
Frequently Asked Questions
Will one use of cannabis show up in a hair test?
It can, but the odds are low. A single session may leave THC-COOH below the 1 pg/mg cutoff. Positive results from one-time use have been reported in cases involving a large dose.
Does hair color affect the result?
Dark hair binds more drug than light hair in lab studies, and that gap has fueled claims of bias in testing. Most labs now use cutoffs low enough to limit the effect, but research on the question is ongoing.
Does shaving your head work?
No. Labs switch to body hair, which carries a longer detection window than scalp hair. A bald donor may also be flagged for tampering or refusal.
How soon after use can a hair test detect cannabis?
About 7 to 10 days. The strand needs time to grow out of the follicle before it carries a readable record.
Do hair tests show how much cannabis you used?
No. A hair test reports a level, not a dose. The number cannot prove how often you used or when the last use happened, though segmented analysis can narrow the timeframe.
Key Takeaways
- Standard detection window: 90 days.
- Window length follows sample length, not the drug.
- Common cutoff: 1 pg/mg of THC-COOH.
- Urine tops out near 30 days, blood near 2 days, saliva near 3 days.
- No product has been proven to beat a hair test.