Cannabis in Hair: How Long Does THC Stay Detectable?

Cannabis in hair: how long it stays comes down to one number, about 90 days. That window is the standard for most labs because head hair grows at about 0.5 inch (1.3 cm) per month, and a typical sample measures 1.5 inches from the scalp. Body hair grows slower, and some labs report traces there for up to 12 months.

How long does cannabis stay in hair after one use?

A single session can show up in hair within 7 to 10 days. THC and its metabolite, THC-COOH, reach the follicle through the bloodstream and settle inside the growing hair shaft. Once that section of hair grows out past the sample the lab cuts, the record of use disappears.

Why do hair tests use a 90-day window?

A hair test reads a timeline, not a single point in time. Labs cut a 1.5-inch sample from the back of the head, which covers three months of growth. If you stop using cannabis today, the test can still show positive until the oldest positive section grows out and leaves the sample area.

How does cannabis get into hair?

THC travels through the blood to the follicle, where cells build the hair shaft. The body turns THC into THC-COOH, and that metabolite binds to melanin inside the growing hair. The result is a locked record that stays put until that hair is cut or falls out.

Smoke residue on the outside of the hair adds a small amount of contamination. Labs wash each sample before testing to strip surface deposits, which is why secondhand smoke seldom causes a positive.

What changes how long cannabis stays in your hair?

Five factors decide the window for any one person.

• Frequency of use: Daily users build more THC-COOH in the shaft than someone who smoked once. Heavy use can push detection past 90 days in some cases.
• Hair source: Head hair grows fastest and clears soonest. Body hair, armpit hair, and pubic hair grow slower and can hold THC for up to a year.
• Body fat and metabolism: THC stores in fat tissue and releases back into the blood, which feeds the follicle over time.
• Potency and product type: Concentrates, vapes, and edibles deliver more THC than a few puffs of flower.
• Sample length and lab cutoff: A shorter sample covers less time, while a lower cutoff catches smaller amounts.

How sensitive is a hair drug test for THC?

Most labs screen at 1 picogram per milligram (pg/mg) of THC-COOH and confirm at 0.1 pg/mg. One picogram is a trillionth of a gram, so the test reads tiny residues. The lab runs an immunoassay first, then confirms any screen with GC-MS or LC-MS/MS.

Can shampoo, bleach, or detox kits clear THC from hair?

No product has shown a reliable result in controlled testing. Bleach damages the hair shaft and can lower THC-COOH in some samples, but it also breaks down other compounds and can raise the measured level. Labs flag treated hair, and an altered sample can count as a refusal or an automatic positive.

How long does cannabis stay in hair compared with other tests?

• Blood: about 1 to 2 days for THC
• Saliva: 1 to 3 days
• Urine: 3 to 30 days, longer for daily users
• Head hair: up to 90 days
• Body hair: up to 12 months

Hair tests catch long-term habits that urine misses after a month. Urine tests track recent use and shift faster when habits change.

When will a hair test come back negative?

A clean result takes about 90 days from your last use, plus the 7 to 10 days it takes for new growth to enter the hair. Users with high body fat may need more time as stored THC leaves the body. Shaving your head does not help, because labs switch to body hair, which holds traces longer.

Does hair color affect detection time?

Dark hair binds more THC-COOH than light hair because melanin attracts the metabolite. Labs account for this with adjusted cutoffs in some cases, though procedures differ by lab. Gray or white hair holds less of the metabolite, which can lower the signal without erasing it.

Can CBD products cause a positive hair test?

Full-spectrum CBD can contain small amounts of THC, and repeated use may build THC-COOH in hair over time. CBD isolate with no THC should not trigger a positive at standard cutoffs. The risk climbs with daily use of high-dose full-spectrum products.

Does a positive hair test prove current impairment?

No. Hair tests show past exposure, not whether you are high at the moment of collection. THC levels in hair do not match blood THC, which is the marker tied to impairment. A positive result only means THC-COOH crossed the lab cutoff during the 90-day window.

Common questions about cannabis and hair tests

How long does cannabis stay in hair for a one-time user?

Up to 90 days. The sample covers the full length of head hair the lab collects, so the timing of a single session matters less than most people assume.

Does hair grow fast enough to clear THC in 30 days?

No. Head hair grows about half an inch per month, so a 1.5-inch sample always covers a 90-day span. New clean growth does not remove the older section from the sample.

Is a hair test the standard for workplace screening?

Most federal workplace programs use urine, not hair. Some private employers, courts, and treatment programs order hair tests because the window is longer and harder to fool.

Can a hair test show how much cannabis you used?

It shows whether THC-COOH is present above the cutoff, not a precise dose. Levels rise with heavy use, but a single number cannot map back to grams smoked.

The bottom line on cannabis in hair

A hair test reads the past 90 days of use, not your state on test day. Head hair gives a clean result about 90 days after the last session, while body hair can hold traces far longer. No wash, bleach, or detox product removes the metabolite in a way a lab cannot spot.