A hair follicle drug test typically detects drug use going back about 90 days. That figure comes from the standard sample: 1.5 inches of hair cut close to the scalp, which represents roughly three months of growth at the average rate of half an inch per month.1Quest Diagnostics. Hair Drug Testing FAQ The window can reach further with longer samples or body hair, and it has a blind spot for very recent use, so the 90-day answer needs a few qualifiers before you can rely on it.
Why 90 Days Is the Standard Number
When you use a drug, your body breaks it down into metabolites that circulate through your bloodstream and reach the small vessels feeding your hair follicles. New hair cells absorb both the parent drug and its metabolites as they form at the root. Once those cells harden into the hair shaft and grow outward, the chemical record is essentially locked in place. Each half-inch of head hair represents roughly one month of that record.
Labs work from the root end of the sample. The first 1.5 inches gets analyzed; anything beyond that length is discarded for a standard screening. So even if your hair is long, only the portion closest to the scalp counts, and the answer to how far back the test reaches is set by that 1.5-inch cutoff rather than by your total hair length.
The Recent-Use Blind Spot
The 90-day window does not include the last several days before your test. After you use a substance, it takes about 5 to 10 days for the affected hair to grow above the scalp where a collector can cut it.1Quest Diagnostics. Hair Drug Testing FAQ Use two days before the collection almost certainly won’t show up in the hair sample.
This is why employers sometimes pair a hair test with a urine screen. Urine catches the past one to seven days; hair covers the roughly three months before that. Together they close the gap. On its own, hair testing is a poor tool for post-accident or reasonable-suspicion screening, where the question is what you took yesterday.
When the Window Reaches Beyond 90 Days
Ninety days is the industry convention, not a biological ceiling. If a court or an employer requests it, a lab can perform segmental analysis: dividing a longer hair sample into sections, each representing a different time period, to build a timeline of use going well past three months.2Psychemedics. Detecting Substance Use: The Time Frame of Hair Drug Testing This kind of extended analysis is more common in forensic work, custody disputes, and other legal proceedings than in routine employment screening, where the standard 1.5-inch cut is nearly universal.
Body Hair
When someone doesn’t have enough head hair, collectors can take a sample from the chest, arms, legs, or underarms. Body hair grows more slowly and tends to reach a certain length before stopping rather than growing continuously. Because of that slower cycle, a body hair sample can reflect a detection window of up to roughly 12 months.1Quest Diagnostics. Hair Drug Testing FAQ The tradeoff is precision. Body hair growth rates and drug incorporation have been studied less thoroughly than head hair, so pinning down exactly when the drug use occurred is much harder with a body sample. You get a longer look-back, but a fuzzier one.
Shaving your head to avoid the test doesn’t get you out of it. The collector will move to body hair instead. If you’ve shaved everywhere, most testing protocols treat the inability to produce a sample the same way they treat a refusal, and employers generally handle refusals the same as positive results.
What Actually Shows Up in the Window
Being inside the 90-day window doesn’t guarantee that a given instance of drug use will register. Several things influence whether use within the detection period produces a positive.
How Often and How Much
Hair testing works best on repeated use. Heavier, more consistent use deposits higher concentrations of metabolites into the hair shaft. Labs apply specific cutoff levels, and use that falls below those thresholds may not trigger a positive at all. Quest Diagnostics describes hair testing as detecting “a pattern of repetitive drug use” rather than isolated incidents.1Quest Diagnostics. Hair Drug Testing FAQ A single use at a party months ago may or may not appear; weekly use over two months almost certainly will.
Hair Treatments
Bleaching, dyeing, perming, and frequent washing with harsh products can reduce metabolite concentrations in hair by damaging the outer cuticle and letting some trapped compounds leach out. They rarely strip a sample clean. A heavily processed sample with borderline levels could drop below the cutoff, but a strong positive from regular use will usually survive cosmetic treatment.
So-called detox shampoos marketed for beating hair tests don’t hold up in the lab. A study of one popular product, Ultra Clean, found that a single treatment reduced cocaine concentrations by about 5% and THC by about 36%, but did not push any drug below the detection threshold. Every substance originally present remained detectable.3National Center for Biotechnology Information. Effect of the Shampoo Ultra Clean on Drug Concentrations in Human Hair
Individual Variation
Half an inch per month is an average. Some people’s hair grows faster, so their 1.5 inches covers more than 90 days; others grow more slowly and cover less. Hair color and thickness also appear to affect how readily drugs bind to the shaft: darker, coarser hair tends to incorporate higher concentrations of certain drugs than lighter, finer hair, a difference that has drawn scrutiny around potential racial bias in testing.
Environmental Contamination
Time spent around people smoking crack or marijuana can leave trace drug compounds on the outside of your hair. Labs wash every sample with solvents before analysis to remove surface contamination, but washing doesn’t always work completely. In one study of judicial hair samples, roughly 7% of contaminated cases still showed significant drug concentrations in the final wash, suggesting contamination had worked its way deeper into the hair.4ScienceDirect. External Contamination of Hair: Still a Debate?
To separate outside exposure from actual use, labs check the ratio of metabolites to parent compounds, since your body produces specific metabolites that passive exposure alone wouldn’t. This works reasonably well for cocaine, where benzoylecgonine is a reliable marker of ingestion, and less well for amphetamines and MDMA, which lack distinctive metabolites that clearly separate use from contamination.4ScienceDirect. External Contamination of Hair: Still a Debate?
What the Test Looks For
A standard hair drug test is a five-panel screening covering the same drug classes most employers screen for in urine:
- Cocaine and its metabolites
- Marijuana (specifically the THC metabolite)
- Amphetamines, including methamphetamine, MDMA, and MDA
- Opiates, including codeine, morphine, and heroin (detected as 6-acetylmorphine)
- Phencyclidine (PCP)
Expanded panels can add benzodiazepines, barbiturates, oxycodone, methadone, tramadol, and other prescription drugs. A 12-panel hair test, for example, layers several prescription opioids and sedatives on top of the standard five. Which panel gets used depends on the industry, the employer, and sometimes state law.
What Hair Testing Cannot Tell Anyone
Hair testing is a lifestyle test, not an impairment test. It shows patterns of use over weeks and months. It does not show whether you are under the influence right now, and it does not reliably show one-time or very infrequent use. If the question is “was this person high at the time of the incident,” a hair test can’t answer it. If the question is “does this person have an ongoing substance use pattern over the past quarter,” it can.
Where Hair Testing Isn’t Allowed
Hair testing is widely used by private employers, especially in trucking, warehousing, and financial services, but it has legal limits worth knowing before you assume a result will carry weight.
For federally regulated transportation workers under Department of Transportation oversight, including truck drivers, pilots, and railroad employees, hair testing is not authorized. DOT regulations permit only urine and oral fluid specimens screened at certified laboratories.6eCFR. 49 CFR Part 40 – Procedures for Transportation Workplace Drug Testing A DOT-regulated employer can use hair testing for its own internal purposes alongside the required urine or oral fluid test, but it can’t substitute a hair test for the federally mandated one.
SAMHSA has been working on mandatory guidelines to bring hair testing into the federal workplace drug testing framework, but as of early 2026 those guidelines remain in the proposed rule stage. Unresolved scientific questions about environmental contamination are one reason for the delay.7Reginfo.gov. View Rule – Mandatory Guidelines for Federal Workplace Drug Testing Programs (Hair)
A growing number of states also restrict marijuana testing specifically, requiring THC screens to test only for psychoactive metabolites indicating recent use. That effectively rules out standard hair tests for marijuana, since hair captures a non-psychoactive metabolite reflecting past use. These laws are moving fast, so check your state’s current rules before relying on any hair test result to establish marijuana use.