Help & Guidance — Purple Haze MK
How Long Does Weed Stay in Your Hair?
THC metabolites are detectable in scalp hair for up to 90 days after cannabis use. Body hair has an even longer window of 6 to 12 months. Hair tests provide a historical record of use that no other test type can match.
THC metabolites, primarily THC-COOH, remain detectable in scalp hair for up to 90 days after cannabis use. This 90-day window is based on the standard 1.5-inch hair sample used in testing, which represents approximately three months of hair growth at the average rate of half an inch per month. Body hair grows more slowly and can reveal cannabis use going back 6 to 12 months. Unlike urine, blood or saliva tests that detect recent use, hair tests provide a historical record of drug use patterns over an extended period. Crucially, weed does not appear in hair immediately: there is a lag of approximately 7 to 10 days between use and the affected hair growing past the scalp to a testable length. Single use can be detected, not only regular use.
How THC Gets Into Hair: The Biology
When you use cannabis, THC is absorbed into the bloodstream and metabolised by the liver into THC-COOH and other metabolites. These metabolites circulate in the blood and are deposited into the hair follicle through the blood vessels supplying the follicle. As new hair grows from the follicle, the metabolites become physically incorporated into the hair shaft itself, bound to the melanin (the pigment) within the cortex of the hair strand.
The critical point is that once embedded in the hair shaft, the metabolites do not fade or break down over time the way they do in urine or blood. They remain fixed in the hair until the hair is cut or falls out naturally. This is why hair tests can reveal historical use: each centimetre of hair represents approximately two months of growth, and testers can date the approximate timing of drug use by the position of the metabolites along the hair shaft.
The 90-Day Detection Timeline
Day 0 to 7: use not yet detectable in hair
Cannabis is used. THC enters the bloodstream and metabolites begin to be deposited into the hair follicle. However, the affected new hair is still below the scalp surface or has not yet grown to testable length. Hair tests performed within 7 to 10 days of use will not detect that use.
Day 7 to 10: detectable hair begins to emerge
The new hair containing THC metabolites begins to grow beyond the scalp at the standard half-inch-per-month growth rate. Once visible above the scalp, it becomes potentially testable. A lab taking a 1.5-inch sample from the scalp would include hair representing the past three months of growth.
Day 10 to 90: standard detection window
The standard 90-day detection window. Hair covering use from 7 to 10 days up to 90 days before the test will be included in a 1.5-inch scalp hair sample. Any cannabis use within this window that produced sufficient metabolite levels in the bloodstream can be detected. Regular use produces more reliable detection than single use.
Beyond 90 days: depends on hair length and sample taken
If you have long hair, use more than the standard 1.5-inch sample, or body hair is tested (which grows more slowly), detection can extend well beyond 90 days. There is no upper limit set by biology: the metabolites persist in hair indefinitely until that section of hair is cut. The limit is practical — laboratories collect a standardised sample length.
Scalp Hair vs Body Hair: Different Windows
Scalp hair grows at approximately half an inch (1.25 cm) per month, giving the standard 90-day window from a 1.5-inch sample. Body hair — including arm, leg, chest, pubic and underarm hair — grows considerably more slowly. Body hair growth rates vary from approximately 0.1 to 0.3 inches per month depending on the site. When labs use body hair in the absence of sufficient scalp hair, the same 1.5-inch sample can represent 6 to 12 months of use history. This significantly extends the detection window and is particularly relevant for people who shave their heads thinking it will defeat a hair test.
Standard detection window from a 1.5-inch scalp hair sample — the longest of any routine drug test type
Detection window when body hair is used — because body hair grows more slowly than scalp hair
Lag before use becomes detectable in hair — the time needed for affected hair to grow past the scalp to a testable length
Factors That Affect Hair Test Detection
Frequency and amount of use
More frequent and heavier use deposits more THC-COOH into the hair follicle through higher and more sustained blood metabolite concentrations. Regular daily use almost always produces a positive hair test. Single or occasional use can also be detected but at lower metabolite concentrations, meaning it is more dependent on test sensitivity and the cutoff level used by the laboratory.
Hair colour and melanin content
THC-COOH binds to melanin, the pigment in hair. Darker hair contains more melanin and therefore binds and retains more THC metabolites than lighter hair. Research suggests people with dark hair may produce higher readings on hair tests than those with light or grey hair at the same level of cannabis use. This is a known limitation of hair testing that can affect the fairness of results across individuals.
Hair growth rate
The standard detection window assumes average hair growth of half an inch per month. Individual variation in growth rate affects the time represented by a given length of hair. Faster-growing hair moves metabolites further from the scalp faster, potentially pushing older use beyond the 1.5-inch sampling zone sooner than average. Slower-growing hair retains older use within the sample zone longer.
Hair treatments and damage
Bleaching, dyeing, perming and chemical straightening can damage hair structure and potentially reduce THC-COOH concentrations in hair. However, no treatment reliably removes metabolites that are bound within the cortex of the hair shaft. Laboratories are aware of treatment effects and some use corrective procedures or take damage into account in analysis. Cosmetic treatments are not a reliable way to defeat a hair drug test.
Shaving your head does not defeat a hair drug test. Laboratories take body hair from the arms, legs, chest or elsewhere when scalp hair is unavailable. Body hair has a longer detection window than scalp hair. Refusing to provide a sample is typically treated as a positive result in employment and legal testing contexts.
For more on cannabis, THC and what is legal in the UK, visit the Purple Haze MK Help and Guidance Centre.
The only reliable way to ensure a negative hair test result is to abstain from cannabis for at least three months before the test date and allow new, unaffected hair to grow. Detox shampoos, specialist treatments and home remedies marketed for passing hair tests have not been proven to reliably remove metabolites embedded in the cortex of the hair shaft. The metabolites are inside the hair structure, not just on the surface, and cannot be washed out by any topical treatment.
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Frequently Asked Questions
Can a single use of cannabis show on a hair test?
Yes, though it is less reliable than detecting regular use. A single cannabis use deposits some THC-COOH into the hair follicle. Whether it produces a detectable signal depends on the amount used, the potency of the cannabis, the test's sensitivity and the individual's hair melanin content. Hair tests are specifically designed to detect patterns of use, and they perform more reliably for frequent use than for isolated single occasions.
How far back can a hair drug test go?
The standard 1.5-inch scalp sample covers approximately 90 days. If longer hair is available and the tester takes a longer sample, detection can extend proportionally. Body hair can extend detection to 6 to 12 months. In principle, hair retains drug metabolites until it is cut, so very long hair could theoretically reveal use from years ago in the sections furthest from the scalp. In practice, laboratories use standardised protocols and sample lengths.
Do hair tests detect CBD?
Standard hair drug tests screen for THC-COOH, the metabolite of THC, not CBD. Pure CBD use will not produce a positive hair test. However, full-spectrum CBD products can contain trace amounts of THC under UK law. Regular use of full-spectrum CBD products could theoretically deposit trace amounts of THC-COOH in hair, though in most realistic scenarios the quantities would be far below the detection cutoffs used by professional laboratories.
Is a hair drug test more accurate than a urine test?
Hair tests and urine tests are accurate at detecting different things. Urine tests are more accurate for detecting recent use, particularly in the days following cannabis use. Hair tests are more accurate for detecting patterns of use over a longer period. Hair tests have a higher false negative rate for occasional single use than urine tests, but a much longer detection window for regular use. Neither is universally more accurate: they serve different detection purposes.
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