The Short Answer
THC metabolites show up in head hair about 7 to 14 days after a single smoke. A test taken within the first few days after use will most likely come back negative, because the drug has to move from the blood into the hair follicle and then get sealed inside the growing shaft. Labs that push cutoffs lower can catch a single session closer to day 5. Labs that use standard cutoffs may miss a one-time use altogether.
Why Hair Lags Behind Blood, Urine, and Saliva
When you inhale, THC moves into the bloodstream within seconds. Blood and saliva tests can catch it the same day. Urine can hold the metabolite for about 1 to 3 days after a single use. Hair works on a different clock.
The follicle at the base of each hair pulls compounds from the nearby blood supply. THC and its metabolites bind to melanin in the growing strand. Only the part of the hair that has already grown above the scalp can be cut and tested, so the drug must be present in the follicle at the moment that section formed. That lag is why a next-day hair test rarely shows anything.
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What the Test Measures
A hair panel does not look for THC itself. It looks for 11-nor-9-carboxy-THC, the main metabolite, written as THC-COOH. Labs wash the sample, break it down, and run an immunoassay screen followed by mass spectrometry confirmation. Federal workplace guidelines set the confirmatory cutoff at 1 picogram per milligram (pg/mg). Some private labs screen as low as 0.1 pg/mg, which raises the chance of picking up a single session.
How a Hair Test Is Collected
- Confirm the sample type. Most hair panels use head hair. If head hair is too short, collectors switch to body hair.
- Cut a sample about 1.5 inches (3.8 cm) long from the scalp, taken close to the skin at the back of the head.
- Mark the root end so the lab knows which side is the most recent growth.
- Send the sample to the lab for washing and screening.
- Wait for confirmation testing if the screen comes back positive.
Before the Test
- Skip bleach, dye, and relaxers in the weeks prior. These treatments can degrade the sample, and collectors may reject treated hair.
- Know the fallback. If head hair is not available, collectors take body hair, which grows slower and holds a longer, less precise record.
- Bring a list of medications and any prescribed THC products. Legal cannabis use can still trigger a positive result on a workplace panel.
What Changes the Detection Window
- Cutoff level: a 0.1 pg/mg screen catches less drug than a 1 pg/mg screen.
- Amount and potency: a single high-THC dab or edible leaves more metabolite than one low-dose puff.
- Frequency: daily users build a steady signal, so hair can stay positive for the full 90 days a 1.5-inch sample covers.
- Hair color and texture: darker, coarser hair binds more THC, and light hair can be harder to test.
- Body fat and metabolism: THC stores in fat tissue and releases over time.
- Sample length: 1.5 inches covers about 90 days. Shorter samples cover less time.
Single Use Versus Regular Use
One smoke session is the hardest case for a hair test to prove. The amount that reaches the follicle may sit below the cutoff, and the lab may call it negative. Repeated use across weeks pushes the concentration well past the threshold. If you used once and the test is more than two weeks out, a positive is possible. If the test is within a few days, a positive is unlikely.
Body Hair and the 90-Day Window
Head hair grows about 0.5 inch (1.3 cm) per month, so the standard 1.5-inch sample reflects roughly the last 90 days. Body hair grows slower and rests longer, so a body hair sample cannot be tied to a specific week. A single session may show somewhere in that strand without the lab being able to say when it happened.
Practical Takeaways
- A single-use positive tends to appear 7 to 14 days after smoking.
- The record can persist for up to 90 days in head hair.
- A test taken within a few days of one session usually reads negative.