Cannabis is typically detectable in head hair for up to 90 days after the last use. Because hair grows about 1 centimeter per month, a standard hair test reflects roughly the previous 3 months of use rather than the days right before the sample was cut.
How the 90-day window is calculated
THC and its main metabolite, THC-COOH, reach the hair follicle through the bloodstream and through sweat and oil on the scalp. As the follicle builds a new strand, traces of those compounds become locked inside the hair shaft. Labs then take a sample close to the scalp, usually about 1.5 inches (3.9 cm), which lines up with roughly 3 months of growth.
- Hair grows about 0.5 inch (1.2 cm) per month.
- A 1.5-inch sample covers about 90 days.
- A 1-inch sample covers about 60 days.
That is why the answer is a range and not one fixed number. The strand is a timeline, and each segment of it holds a different period of use.
how long is cannabis detectable in hair
What changes your individual result
The 90-day figure is an outer limit, not a guarantee either way. Several things shift where a person lands inside that window.
Cannabis Half-Life and Body Fat: How Long THC Stays Stored
- How often and how much you use. Daily or heavy use builds higher concentrations than a single session, which makes traces easier to confirm.
- Potency of the product. Higher-THC flower, concentrates, and edibles deliver more material to the follicle.
- Your metabolism and body composition. THC stores in fat tissue and releases over time, so slower clearance can extend exposure.
- Hair color and melanin. Melanin binds drug compounds, and darker hair tends to hold more of them.
- Hair treatments. Bleaching, dyeing, and relaxing can lower measured levels, but the effect is unpredictable and not something to count on.
- Growth rate. Age, genetics, and health all change how fast your hair grows.
Head hair versus body hair
Head hair is the standard sample because its growth rate is steady and well documented. Body hair grows more slowly and stays in the growing phase longer, so a body hair test can reach back well past 90 days, sometimes a full year. If head hair is not available, labs may use body hair or fingernails, and both tend to widen the detection window rather than shrink it.
What "detectable" actually means at the lab
A hair test does not report every trace it finds. Labs compare the result against a cutoff concentration, and anything below that line is reported as negative. Federal workplace guidelines set specific cutoff values for the marijuana metabolite in hair, with a screening cutoff and a lower confirmation cutoff. This is why two people with similar use habits can get different results: the question is not whether any compound is present, but whether enough of it crossed the lab's threshold.
Can you remove THC from hair?
No shampoo, detox kit, or home remedy has been shown to clear metabolites from inside a hair strand. The compounds are bound within the shaft, so surface washing reaches only the outside. Bleaching and chemical straightening can reduce readings in some studies, but results vary by hair type and treatment, and none of these methods is reliable against a lab test.
Where convenience fits in
At-home hair tests have made screening easier to arrange, and that ease is a big part of why they get used. Convenience has limits, though. A home kit is a screening tool, and any result that matters, whether for a job, a legal matter, or a treatment plan, needs confirmation from an accredited lab using a validated method.
Key takeaways
- Head hair: up to about 90 days after last use.
- Body hair: often longer, sometimes up to a year.
- Frequency, potency, melanin, and hair treatments all move the result.
- Lab cutoffs decide what counts as positive, not the presence of any trace amount.