Cannabis in Hair: How Long THC Stays Detectable

Cannabis in hair is usually detectable for up to 90 days after the last use. That three-month figure is the window most hair drug tests are built around, although occasional users may fall closer to 30 days, and daily, heavy users can sometimes be flagged a little longer. So when people ask cannabis in hair how long it stays, the practical answer for head hair is roughly three months.

Why Hair Holds THC So Long

When you smoke, vape, or eat cannabis, THC is broken down into metabolites such as carboxy-THC. As hair grows, trace amounts of those compounds are incorporated into the follicle and become sealed inside the hair shaft. Blood and saliva give a snapshot of the moment. Hair does not. Once a metabolite is inside the shaft, it stays there as that strand grows out and eventually gets cut.

Hair Growth Sets the Timeline

The detection window is less about chemistry and more about arithmetic.

• Head hair grows about half an inch, or roughly 1.25 cm, per month.
• A standard sample of about 1.5 inches taken from the scalp covers roughly 90 days of growth.
• Labs can test shorter segments to estimate when use happened, but the estimate is broad.
• A longer sample can reach back further, which is why very long hair does not automatically mean a longer positive.

Head Hair vs. Body Hair

Body hair grows more slowly than scalp hair, so it can hold metabolites for longer and reflects a wider, blurrier timeline. Body hair also picks up external contamination more easily. Most labs prefer head hair for those reasons, and if head hair is not available they will usually collect body hair instead of skipping the test.

What Changes Your Personal Window

• Frequency and amount of use. Daily use builds higher concentrations in the shaft than a single session.
• Product potency. High-THC concentrates and edibles deliver more THC per session than a low-dose joint.
• Hair color and texture. Some research suggests darker, coarser hair binds certain compounds more readily.
• Hair treatments. Bleaching, relaxing, and repeated dyeing can damage the shaft enough to produce an inconclusive result rather than a clean one.
• The cutoff level. Labs set a threshold, and a higher cutoff means a shorter positive window.
• Sample length and location. A 1.5-inch head sample and a body hair sample are not equivalent.

Can a Hair Test Detect Use From Last Week?

Usually not. Newly grown hair needs roughly 7 to 10 days to emerge far enough from the follicle to be collected and tested. That makes hair a backward-looking test. It is good at showing a pattern across weeks or months, not at confirming what happened on a Friday night. Urine and saliva are better suited to recent use.

How Hair Compares With Other Tests

• Blood: shortest window, generally hours to a couple of days.
• Saliva: hours to about a day or two, sometimes a bit longer with heavy use.
• Urine: days for one-time use, and up to a month or more for chronic heavy use.
• Hair: the longest window, commonly cited as up to about 90 days.

Common Myths About Hair Testing

• Detox shampoos. No shampoo is proven to remove metabolites from inside the hair shaft. Marketing claims run well ahead of the evidence.
• Bleaching or relaxing. This can damage protein structure and sometimes triggers an invalid or unsuitable sample, which does not read as a pass.
• Secondhand smoke. Labs wash samples before analysis and compare the parent drug to its metabolite, which helps separate exposure from actual use.
• Shaving. Body hair can be collected instead, and body hair has a longer, less precise window.

What a Hair Test Cannot Tell You

A positive hair result shows that cannabis metabolites were present in the sample. It does not measure how much you used, how often, or whether you were impaired at any point. Cutoff levels, lab methods, and sample quality all shape the result, and a negative test does not prove zero use over the whole period either.

The Bottom Line

For most people, cannabis in hair is detectable for about 90 days, driven by how fast head hair grows and how much of a sample is collected. Light or one-time use tends to clear sooner, and long-term heavy use can sit at the far end of that range or slightly beyond. If timing matters for a specific test, ask the testing lab which specimen, cutoff, and sample length they use, since those details decide the real answer.