Short answer
In most cases, cannabis is detectable in head hair for up to 90 days after last use. That 90-day figure is the standard detection window for a hair follicle test. The exact number depends on how much and how often you used, how fast your hair grows, where the hair sample comes from, and the cutoff level used by the lab. A single use may clear the cutoff sooner. Very recent use, such as within the past week, may not show up yet because the drug needs time to enter the hair follicle and grow out with the hair shaft.
do hair detox shampoos work for cannabis
Why hair tests use a 90-day window
Head hair grows about 0.5 inch (1.25 cm) per month. A 1.5-inch sample taken from the scalp near the skin can represent roughly the last 90 days of use. Labs often test the first 1.5 inches of hair from the root. If the hair is longer, a lab can cut it into segments and estimate when use happened. This is one reason hair tests are used for a longer look back than urine or saliva tests.
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What can shorten or lengthen the window
- Use pattern: Daily or near-daily use can build up more drug in the hair and stay detectable for the full 90 days or longer. One-time or rare use may fall below the cutoff in less time.
- Hair growth rate: Slow hair growth can stretch the window. Fast growth may move the drug out of the tested section sooner.
- Hair type and location: Head hair has a fairly predictable growth rate. Body hair grows slower and can have a different detection window. A body hair sample may reflect a longer or less clear timeline.
- Test cutoff: A lab with a lower cutoff can detect smaller amounts. A higher cutoff may report a negative sooner.
- External contamination: Smoke, residue, or direct contact can put cannabis on the outside of the hair. Labs may use wash steps, but contamination is still a known issue.
- Hair treatments: Bleaching, dyeing, and relaxing can damage hair and affect results, but they do not reliably remove drugs. Results can be unpredictable.
Head hair vs body hair
Most hair follicle tests use head hair. If head hair is too short or unavailable, a lab may use body hair. Body hair grows more slowly than scalp hair, so it can hold a drug for a longer time or give a less precise timeline. Arm, leg, chest, or underarm hair can be useful when scalp hair is not an option, but the 90-day rule is based on head hair.
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If you are buying an at-home hair test
Look for a test that uses a laboratory, not an instant strip. A valid hair test should include a chain of custody, a clear cutoff level, and confirmation by LC-MS/MS or GC-MS/MS when screening is positive. Check how the kit collects the sample. Most need a small lock of head hair cut close to the scalp. Read the instructions before you buy. A kit that claims to test all drugs with one hair strand may not be accurate. Also check whether the lab reports THC only or includes other cannabinoids. For legal or employment reasons, an at-home result may not be accepted. A certified lab result is the standard.
Pitfalls to avoid
- Detox shampoos and masks do not have proof that they remove THC from hair. Some can damage hair and create false results.
- Bleaching or dyeing hair is not a reliable way to beat a hair test.
- Testing too soon after use can give a negative even if a later test would be positive.
- Body hair and head hair can give different answers. Do not assume they match.
- Secondhand smoke exposure can contaminate hair, but it rarely causes a positive at standard cutoffs. The risk is higher with heavy exposure.
FAQ
Can one use of cannabis show up in a hair test?
It can, but it is less likely than with heavy use. A single use may fall below the lab cutoff, especially if the test is done soon after use or if the cutoff is high.
Can a hair test detect cannabis from yesterday?
Usually no. Hair needs time to grow and carry the drug into the tested section. Urine or saliva tests are better for recent use.
Does hair length change the detection window?
Yes. A 1.5-inch sample from the root covers about 90 days. Longer hair can be segmented to look further back, but the standard window stays around 90 days from the root.
Can body hair detect cannabis longer than head hair?
Body hair grows slower, so it may reflect a longer window. It can also be harder to interpret. Most labs prefer head hair when it is available.
Do detox products work for hair tests?
There is no proven shampoo or treatment that removes THC from hair. Do not rely on detox products for a hair follicle test.