The short answer for most people is 90 days. That figure comes from the standard hair follicle test used by most labs: a 1.5-inch sample of head hair, cut close to the scalp. Head hair grows about 0.5 inch per month, so 1.5 inches covers roughly three months of growth. Labs test for THC-COOH, a metabolite that binds to the hair shaft as it forms. The 90-day window is the top pick because it is the most widely cited and standardized estimate. It assumes head hair, a lab that follows common cut-off levels, and no unusual factors that shorten or extend detection.
Cannabis Metabolites in Hair: What Impacts to Know
Head hair: the 90-day standard
Head hair is the default sample for pre-employment, legal, and clinical hair tests. It offers a predictable timeline and a clear collection method.
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- Pros: The 90-day window is consistent across major labs. Collection is quick and non-invasive. It can show a pattern of use over months, not just days.
- Cons: It cannot tell exactly when you used cannabis within those 90 days. External contamination from smoke or surfaces is possible, though labs use wash steps to reduce it. A positive result does not prove impairment.
Use this section if you need a general answer: for a single-use or occasional user, 90 days is the safe assumption for head hair.
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Body hair: longer but less precise
Body hair grows more slowly than head hair, often 0.25 to 0.5 inch per month. That slower growth means a body hair sample can reflect a much longer history, sometimes 6 months or more. However, the exact window is harder to pin down.
- Pros: It can detect cannabis use over a longer period when head hair is too short or unavailable. It may be useful for a broad historical snapshot.
- Cons: The timeline is less predictable. Labs may report different windows. It is harder to link a positive result to a specific time frame.
Use this section if you have very short head hair or need to understand why some sources say cannabis can stay in hair for 6 months or longer.
What changes the detection window
Several factors can push your personal window toward the short end or the long end. Most of them are outside your control once cannabis is in your system.
- Pros of understanding these factors: you can set a realistic expectation and avoid relying on detox claims.
- Cons: you cannot change most factors, and no proven method removes THC-COOH from hair.
- Frequency and dose: Daily or high-potency use tends to deposit more THC-COOH and can extend detection.
- Hair color and texture: Dark hair may bind more metabolite than light hair in some studies, though results vary.
- External exposure: Heavy smoke in a closed room can contaminate hair. Labs wash samples, but contamination remains a known issue.
- Cosmetic treatments: Bleaching, dyeing, and relaxing can reduce detectable THC-COOH, but they do not reliably produce a negative result.
- Individual metabolism: Body weight, liver function, and genetics affect how you process THC.
Use this section if you are a heavy user: plan for the longer end of the range, especially with body hair.
How hair testing compares with other tests
Hair testing has the longest detection window for cannabis. Urine typically detects THC-COOH for up to 30 days after last use, though heavy users may show positive for longer. Blood detection is usually measured in hours to a few days. Saliva tests often detect cannabis for 1 to 3 days.
- Pros of hair testing: it covers a long history and is difficult to cheat with short-term abstention.
- Cons: it does not measure impairment, and external contamination is a known limitation.
Use this section if you need to compare a hair test with a urine or saliva test. If you are facing a hair test, plan for the 90-day window, and consider the body hair exception if head hair is not available.
What to do with this information
If you use cannabis occasionally, assume 90 days for head hair. If you use daily or heavily, assume the longer end, and remember that body hair can extend the window to 6 months or more.
- Pros of a conservative estimate: you avoid surprises and can plan around the longest likely window.
- Cons: it may be longer than your actual window, which can cause unnecessary worry.
Legal purchase from a licensed retailer does not change how your body stores THC-COOH. Testing policies are set by employers, labs, and courts, so check the specific rules that apply to you. No detox product has been shown to reliably beat a hair follicle test.