Cannabis
How Long Does Cannabis Stay in Your Hair? A Workplace Drug Test Guide
Psychologist | Specialist Writer in Psychology & Behavioural Science
Sep 22, 2025
Facing a pre-employment hair follicle drug test can be a source of significant anxiety. Unlike urine or saliva tests that measure recent use, a hair test is designed to look back months, creating a long-term record of consumption. Understanding how this test works, its detection window, and its limitations is the first step to navigating this stressful situation.
One of the most important things to know is that hair analysis doesn’t just capture isolated use — it provides a broad, historical picture of drug patterns. In fact, research comparing hair analysis with self-reported drug use in emergency department patients found that hair testing often detected use that participants under-reported, confirming its reliability for workplace and forensic contexts.¹ This explains why employers and courts rely on hair testing: it is difficult to cheat and cannot be outsmarted by short-term abstinence, since while abstinence prevents new metabolites from being incorporated into growing hair, previously formed segments continue to contain THC-COOH until they are naturally shed or cut away.
This guide provides a science-based explanation of hair drug testing for cannabis. We will demystify the process, explore the factors that influence results, and give a realistic assessment of the so-called "detox" methods, equipping you with facts, not fiction.
Direct Answers
How long does cannabis stay in hair? Typically up to 90 days for head hair, and sometimes much longer for body hair.
How does it get in there? A THC metabolite called THC-COOH is deposited from the bloodstream into the hair follicle as it grows.
Can one-time use be detected? It’s possible, but less likely than for regular use. Tests are designed to detect patterns of consumption over time.
Do detox shampoos work? No. Scientific evidence overwhelmingly shows that shampoos cannot remove metabolites from inside the hair shaft.
Is a hair test accurate? Yes. It is a highly accurate, legally defensible method for detecting historical cannabis use, which is why it is widely used in employment and forensic settings.
Key Takeaways
A hair follicle test provides a long-term history of cannabis use, typically looking back over 90 days.
The test detects THC-COOH, a non-psychoactive metabolite, meaning it identifies past use, not current impairment.
The frequency of cannabis use is the single biggest factor determining whether a test will come back positive.
So-called detox shampoos are ineffective, as they cannot remove drug metabolites embedded deep within the hair shaft.
The only guaranteed way to pass a hair drug test is to abstain from cannabis for at least 90 days, allowing new, clean hair to grow.
How Does a Hair Follicle Drug Test Actually Work?
When cannabis is consumed, the body metabolises THC into 11-nor-9-carboxy-THC (THC-COOH), a non-psychoactive compound. This metabolite enters the bloodstream and becomes deposited at the base of the hair follicle. As the hair grows, THC-COOH is incorporated into the cortex — the inner structure of the hair strand — effectively locking in a chemical record of use.
Think of a strand of hair like a tree trunk with growth rings: just as rings capture environmental history, your hair captures a timeline of what has been in your bloodstream. Standard workplace testing analyses the 1.5 inches closest to the scalp, reflecting roughly 90 days of growth.
Recent advances in toxicology have made this process even more sensitive. Using techniques such as electromembrane extraction, researchers can isolate drugs of abuse from finely pulverized hair samples with remarkable accuracy.² This innovation confirms why hair analysis is considered a forensically robust method: it detects drug use with high precision and is extremely resistant to tampering.
The Science of Hair Testing: THC-COOH, Cut-off Levels, and False Positives
Hair testing is regarded as one of the most scientifically robust drug-screening methods, primarily because it targets THC-COOH, not THC itself. THC-COOH is a non-psychoactive metabolite that only appears in hair after cannabis has been metabolised inside the body. This means external contamination (like sitting in a smoky room) cannot cause THC-COOH to appear in the cortex of the hair shaft.
A landmark study highlighted the problem with testing for THC directly: cannabinoids from smoke or surface contact can sometimes be detected on the outside of hair strands, but this does not prove actual cannabis use.³ This is why modern forensic protocols focus on THC-COOH — to distinguish real consumption from passive exposure.
To ensure fairness, laboratories also use cut-off thresholds. For cannabis, the screening cut-off is typically 1 picogram per milligram (pg/mg). Anything below this level is reported as negative, meaning casual or one-off exposures that don’t reach the threshold won’t cause a positive result. Additionally, every hair sample is washed extensively before analysis, further eliminating the chance of false positives from environmental contamination.
This multi-layered process — targeting THC-COOH, enforcing strict cut-offs, and washing samples — makes hair testing not only highly accurate but also legally defensible. It is why employers, courts, and sporting bodies continue to rely on it as a gold-standard tool.
The 90-Day Detection Window: What Does It Really Mean?
The 90-day detection window is considered the industry standard in workplace drug testing. On average, human hair grows about 0.5 inches per month, so testing the 1.5 inches closest to the scalp captures roughly three months of cannabis use history.
It’s important to note that the test does not detect very recent use. It typically takes 7–10 days after consumption for THC-COOH metabolites to be incorporated into newly grown hair above the scalp. This means that if you consumed cannabis yesterday, a hair test taken today would almost certainly not detect it.
What the test does capture is patterns of use over time. A study comparing hair samples with self-reported cannabis consumption found that heavy, light, and non-users showed distinct cannabinoid profiles in hair, with heavy users consistently testing positive at levels far above cut-off thresholds.⁴ This reinforces that hair testing is not designed to catch a single experiment, but to reveal repeated or chronic use.
For employers, this makes hair analysis attractive: it tells them not just whether an applicant has used cannabis, but whether their use reflects an ongoing lifestyle pattern. For individuals, it means that one-off or occasional use is less likely to be detected than a daily habit — though detection is never impossible.
What Factors Can Influence Your Test Results?
While the 90-day window is the baseline standard, the actual amount of cannabis metabolite detected in hair can vary depending on multiple biological and environmental factors. These don’t determine whether THC-COOH is present — they influence how much is deposited and whether it crosses the cut-off threshold for a positive result.
Frequency of Use (Most Critical): Regular, heavy use leaves a much stronger THC-COOH signal in the hair than one-time or occasional use. A single experiment may not even register above the cut-off, while a chronic user will almost certainly test positive.
Hair Colour and Melanin Content: Darker hair contains more melanin, and THC-COOH is known to bind to melanin pigments. As a result, individuals with black or brown hair may retain higher concentrations of cannabinoids than blond or light-haired individuals with the same usage pattern.
Metabolism and Body Fat: THC is fat-soluble. People with a higher body fat percentage can store more THC, which is slowly released back into the bloodstream even after cessation, continuing to deposit metabolites into hair over time.
Bioanalytical Variables: A review of cannabis detection techniques in hair emphasised that factors like sample preparation, washing protocols, and lab methodology can influence measured concentrations, even when the same individual’s hair is tested.⁵
These variables underscore why two people with the same use history may receive different lab results. Still, the dominant factor is frequency of use — the single most reliable predictor of whether a hair test comes back positive.
Hair Test vs. Other Drug Tests: A Comparison
The key advantage — and drawback — of hair testing is its long detection window. Unlike urine, saliva, or blood tests, which capture recent or current use, hair testing provides a historical overview of drug consumption.
A large-scale toxicology study analysing 1,865 hair samples from individuals in abstinence programs demonstrated the sensitivity of modern LC–MS/MS techniques. Researchers were able to detect cannabinoids and other psychotropic drugs long after they had cleared from blood or urine, confirming hair’s value as a long-term archive of drug use.⁶
Table 1: How hair compares with other common test types
Test Type | Detection Window (Chronic User) | What It Detects | Primary Purpose |
Hair | Up to 90 days (head hair); up to 12 months (body hair) | History of use | Pre-employment, forensic/legal |
Urine | Up to 30 days | Recent use | Most common workplace screening |
Saliva | 24–48 hours | Very recent use | Roadside checks, on-site screening |
Blood | 1–2 days | Current impairment | Accident investigations, DUI cases |
This comparison highlights why employers and courts often rely on hair analysis. It is nearly impossible to “time” your way around the test, unlike urine or saliva testing where abstinence for a short period can make the difference between a pass and fail.
The Big Question: Can You Cleanse Your Hair to Pass a Test?
One of the most common myths around workplace hair testing is that “detox” shampoos or home remedies can remove cannabis traces from hair. The science says otherwise.
Drug metabolites like THC-COOH are deposited inside the cortex of the hair shaft as it grows. This means they are locked into the internal structure, not simply sitting on the surface. Commercial shampoos may remove oils or contaminants on the outside, but they cannot reach the inner cortex where the metabolites reside. Laboratories also wash every sample extensively before analysis, rendering any external treatment even less effective.
Recent research confirms just how firmly usage patterns show up in hair. A 2024 study found that hair-derived cannabinoid levels correlated strongly with both self-reported cannabis use and cannabis-related problems,⁷ indicating that the test measures real, embedded biological evidence rather than external contamination. In other words, shampoos cannot rewrite a chemical history already embedded in the hair.
For heavy or chronic users, no cosmetic product has been shown to reliably reduce THC-COOH concentrations below forensic cut-off thresholds. The only proven way to “cleanse” your hair is time: abstaining long enough for new, clean hair to grow from the scalp.
The Truth About “Detox” Shampoos and Home Remedies
The market is full of shampoos and “home recipes” that promise to flush cannabis from your hair. Scientifically, these claims do not hold up.
Forensic labs now employ micro-segmental hair analysis, a technique capable of analysing hair in millimetre sections. This allows toxicologists to reconstruct a person’s drug use history with extraordinary precision.⁸ When labs can pinpoint individual weeks of exposure within a strand of hair, it becomes clear why shampoos are ineffective: the drug record is embedded deep in the hair’s structure, far beyond the reach of surface cleansers.
Clinical data support this conclusion. In one study monitoring patients after treatment protocols, hair analysis reliably tracked drug use and abstinence over time. Even when individuals attempted to alter their hair through cosmetic treatments, drug metabolites remained detectable, confirming that chemical history cannot be erased from the cortex.⁹
Together, these findings dismantle the detox myth. At best, aggressive treatments like bleaching may reduce concentrations by 40–60%, but rarely enough to drop a chronic user below the forensic cut-off. And such treatments cause significant hair damage, often raising suspicion rather than avoiding detection.
Can Bleaching or Dyeing Your Hair Help?
Some people turn to hair bleaching or dyeing in the hope of masking cannabis use. While harsh chemical treatments can indeed reduce drug metabolite levels, they are far from foolproof.
Studies of ethyl glucuronide (EtG), an alcohol biomarker incorporated into hair in a manner similar to drug metabolites, show that bleaching and other oxidative treatments can markedly reduce concentrations—often by 50% or more—and in some cases nearly eliminate them entirely.¹⁰ In practice, this reduction is rarely enough to drop a chronic user below forensic cut-off thresholds. Forensic labs are also trained to recognize hair that has been chemically altered, and suspicious samples may undergo more rigorous testing.
Furthermore, repeated bleaching or dyeing causes significant structural damage to the hair shaft, often making it more brittle and porous. Ironically, this can sometimes make drug residues easier to detect, not harder.
The takeaway is clear: while cosmetic treatments may alter test results slightly, they do not provide a reliable way to pass. In fact, tampering attempts often raise red flags in forensic contexts and may even count as a failed test if manipulation is suspected.
Expert Tip from a Toxicologist: Why You Can’t Outsmart the Test
The idea of “cleansing” your hair is a persistent myth, but forensic toxicologists emphasize that it misunderstands how hair testing works. The metabolite THC-COOH is incorporated into the hair’s cortex during growth — it is part of the structure, not a residue sitting on the surface.
Modern laboratories use dynamic multiple reaction monitoring (MRM) with LC–MS/MS to detect drugs of abuse in hair with extreme precision. In one study, this approach enabled rapid, simultaneous detection of multiple drugs, proving that even complex or damaged hair samples still reveal their chemical history.¹¹ This level of sensitivity explains why shampoos, dyes, or home remedies are ineffective: the metabolites remain inside the hair, and modern instruments can find them.
Expert Tip:
"The drug metabolite is not just on the hair; it is inside the hair. Short of shaving your head — which itself raises immediate suspicion — there is no reliable way to remove it. Any claims to the contrary are marketing, not science."
For anyone facing a hair test, this means the only guaranteed solution is time and abstinence. Attempting to manipulate the sample not only fails scientifically but may also trigger closer scrutiny in a forensic or employment setting.
External Contamination vs. True Use
One of the most common worries about hair drug testing is the risk of false positives from external contamination — for example, secondhand smoke or environmental contact. Forensic toxicologists have studied this extensively, and the findings are clear: when proper laboratory protocols are used, positive results reflect true drug use, not surface contamination.
A forensic evaluation of external heroin contamination demonstrated that even when hair was exposed to drug powder, rigorous washing and metabolite targeting distinguished contamination from actual ingestion.¹² The same principle applies to cannabis: because labs test for THC-COOH, which only forms inside the body, passive exposure cannot create a positive result.
Hair testing is also valuable beyond single substances. A 2021 review showed that hair analysis reliably detects polydrug use, making it an indispensable forensic and workplace tool for monitoring multiple drugs over long periods.¹³ This reinforces why cannabis users cannot rely on the “secondhand smoke” excuse — if a sample tests positive for THC-COOH, it means ingestion occurred.
In practice, this makes hair testing both highly accurate and legally defensible, providing employers, courts, and clinicians with confidence in the results.
Table 2: Comparison of Drug Testing Methods
Test Type | Detection Window | What It Detects | Strengths | Limitations |
Hair | ~90 days (head hair); up to 12 months (body hair) | Long-term history of cannabis use (THC-COOH) | Longest detection window; legally defensible; very difficult to cheat | Cannot detect very recent use (7–10 day lag); more costly |
Urine | 1–30 days (depending on frequency of use) | Recent use (THC metabolites excreted) | Inexpensive; widely available; good for routine screening | Easy to manipulate with timing/hydration; shorter detection window |
Saliva | 24–48 hours | Very recent use (parent THC) | Non-invasive; good for roadside/on-site testing | Very short detection window; less reliable for chronic users |
Blood | 1–2 days | Current impairment (active THC) | Direct measurement of active THC; gold standard for accidents/DUI | Highly invasive; expensive; shortest detection window |
Frequently Asked Questions (FAQs)
1. How long does cannabis stay in your hair?
Typically up to 90 days for head hair. For body hair, which grows more slowly, the detection window can extend to 12 months or more.
2. Can one-time cannabis use be detected in a hair test?
It is possible, but less likely. Hair tests are designed to identify patterns of use. A single occasion may not leave enough metabolite to cross the forensic cut-off threshold.
3. Will secondhand smoke make me fail a hair test?
No. Labs look for THC-COOH, a metabolite only formed after cannabis is processed inside the body. External smoke cannot deposit this metabolite into hair. Samples are also washed before testing, removing contaminants.
4. Do detox shampoos work for hair tests?
No. Shampoos can only clean the outside of the hair shaft, while THC-COOH is locked deep inside the cortex. Scientific evidence shows they are ineffective, and labs wash samples before analysis anyway.
5. Does bleaching or dyeing hair guarantee a negative result?
No. While bleaching may reduce metabolite levels by up to 40–60%, it rarely eliminates them completely. Damaged hair may also draw extra scrutiny in forensic testing.
6. Will CBD products cause a positive hair test?
Pure CBD isolate should not. However, many “full-spectrum” or mislabeled products contain THC. Using unregulated CBD products carries a real risk of testing positive for THC-COOH.
7. What is the only guaranteed way to pass a hair drug test?
Complete abstinence for at least 90 days before the test. This allows new, clean hair to grow free of THC-COOH, while old hair containing metabolites is cut away over time.
Glossary of Key Terms
THC-COOH: A non-psychoactive metabolite of THC that becomes trapped inside the hair shaft and serves as the forensic target in testing.
Cut-off Level: The laboratory threshold for reporting a positive result; for cannabis hair tests, usually 1 pg/mg.
Hair Cortex: The inner part of the hair strand where metabolites are permanently stored.
LC–MS/MS: A highly sensitive laboratory technique (liquid chromatography–tandem mass spectrometry) used to detect drugs in hair.
Micro-Segmental Analysis: Forensic method dividing hair into millimetre sections to track week-by-week drug use history.
External Contamination: Drug particles deposited on the hair surface (e.g., smoke or dust). Proper washing and metabolite targeting eliminate this as a cause of positives.
Detection Window: The period of time a test can identify drug use. For hair, typically 90 days or longer.
Conclusion: A Test of History, Not a Test of Tricks
The hair follicle drug test is one of the most powerful tools available to employers, forensic labs, and courts. Unlike urine, saliva, or blood tests, which capture only recent or current use, hair testing provides a three-month historical record — and for body hair, even longer.
The science is clear:
Hair tests target THC-COOH, a metabolite that proves actual cannabis consumption.
External contamination and secondhand smoke do not cause positives.
Detox shampoos, dyes, and home remedies cannot erase the metabolite record once it is embedded in the cortex.
The only guaranteed way to pass is abstinence and allowing new, clean hair to grow.
For those facing workplace screening, the most empowering tool is not a product but knowledge. By understanding how hair testing works and why its results are so difficult to alter, you can approach the process with clarity — and avoid the false hope sold by ineffective quick fixes.
References
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