Cannabis in Hair: How Long It Stays Detectable

How long does cannabis stay in hair?

A scalp hair drug test can detect cannabis for up to 90 days after your last use. Labs usually test the first 1.5 inches (3.9 cm) of hair measured from the scalp, which tracks about three months of growth at roughly 0.5 inch per month. A single session can show up in as little as 5 to 7 days. Body hair grows slower, so traces there can last 6 to 12 months or longer.

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Why hair holds cannabinoids for months

Blood carries THC and its breakdown products to the follicle while the strand is forming. Those compounds get locked into the keratin core as the hair hardens. Once the strand leaves the scalp, blood flow, sweat, and shampoo no longer reach the material inside it. That is why a hair test reads as a timeline of use rather than a snapshot of the past few days.

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Most panels screen for THC and its metabolite carboxy-THC (THC-COOH). A confirmed positive requires a second analysis, usually chromatography paired with mass spectrometry.

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Detection windows by hair type

Scalp hair

• First 1.5 inches: up to 90 days back
• Single use: detectable around 5 to 7 days
• Longer segments of 3 to 6 inches: 6 to 12 months back

Body hair

• Chest, arm, leg, and pubic hair grow slower than scalp hair
• Typical window: 6 to 12 months
• Collected when scalp hair is shorter than 1.5 inches or damaged

Estimate your own window

You need a tape measure, a mirror, and the date of your last use.

1. Pick the crown of your head as the sampling point, since that is where labs cut closest to the scalp.
2. Measure 1.5 inches out from the scalp along one strand.
3. Treat that first 1.5 inches as the past 90 days.
4. Add one month for every extra 0.5 inch of length.
5. Compare that timeline with the date of your last use.
6. Switch to body hair for the estimate if your scalp hair is shorter than 1.5 inches.

What does not clear THC from hair

• Detox shampoos and follicle cleansers
• Bleach, dye, or relaxers
• Vinegar, baking soda, and cayenne rinses
• Extra water, sweating, or exercise

Harsh chemical treatments can damage the strand and produce an inconclusive result, but labs then retest the sample or fall back to body hair. No at-home product has been shown to strip cannabinoid metabolites from hair in a way that holds up under lab confirmation.

What changes the detection window

• Use frequency: daily users carry higher concentrations than occasional users
• Dose and potency: higher THC products leave more marker behind
• Growth rate: age, hormones, and health shift how fast hair grows
• Melanin content: darker, coarser hair binds more residue in some studies
• Cutoff level: a lab's threshold decides how far back a sample reads positive
• Segment length: cutting more hair means a longer look back

US testing rules worth knowing

Hair is not one of the specimen types in the federally mandated workplace drug testing panels, which rely on urine and oral fluid. Private employers, courts, and treatment programs can still order a hair test under their own policies. Some states limit or ban hair testing for marijuana in hiring decisions, so check your state law before you assume a hair test applies to you.