Hair tests look for THC-COOH (carboxy-THC), the main metabolite the body makes after THC enters the blood. What impacts cannabis metabolites in hair comes down to a short list: dose, how often you use, hair color and texture, growth rate, body fat, hair treatments, and outside contamination from smoke or residue. A standard scalp hair test covers about 90 days of use.
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What Are Cannabis Metabolites in Hair?
After you inhale or eat THC, the liver breaks it into several compounds. The one labs target most is 11-nor-9-carboxy-THC, written as THC-COOH. That molecule is not psychoactive, but it lingers in the body longer than THC itself.
How Long Does CBD Stay in Hair?
Metabolites reach hair through two paths. Blood vessels feeding the follicle carry them into the growing strand, and sweat, sebum, and smoke can deposit them on the outside. Both paths leave traces that a lab can pick up.
how many months does thc stay in hair
THC-COOH vs. Other Markers
Some labs also screen for parent THC, cannabinol, and cannabidiol. THC-COOH carries the most weight because it indicates the body processed THC. A positive CBD result alone does not mean someone used cannabis with high THC.
What Impacts Cannabis Metabolite Levels in Hair?
No two people store the same amount. The factors below explain most of the gap between a light user and a heavy one, and between one strand and another.
Dose and Frequency of Use
One low-dose session may leave nothing above the cutoff. Daily use over months builds higher concentrations per strand. Concentration tracks total intake, so a heavy user reads higher than a weekend user on most tests.
Hair Color, Texture, and Melanin
Melanin binds drug compounds. Dark, coarse hair holds more metabolite than blond or fine hair, all else being equal. Some labs apply different cutoffs by color, a practice that has faced court challenges in the US.
Hair Growth Rate and Length
Scalp hair grows about 0.5 inch (1.3 cm) each month. A 1.5-inch sample covers about 90 days of use. Body hair grows at a slower, less predictable rate, so labs prefer scalp samples when they can get them.
Body Weight, Metabolism, and Age
THC is fat-soluble and stores in fat tissue. People with more body fat can release metabolite into the blood over a longer stretch. Liver and kidney function shape how fast the body clears THC-COOH, and both change with age.
Hair Treatments and Products
Bleach, relaxers, perms, and repeated dye can strip or break down metabolite inside the strand. A treated sample may read lower than an untreated one from the same person. Results vary with the product, the number of applications, and how damaged the hair is.
External Contamination
Secondhand smoke, handling flower, and touching resin-coated surfaces can put THC on the outside of hair. Labs wash samples before testing to cut this effect. Washing does not remove everything, so some labs test the wash liquid to check for contamination.
Time Since Last Use
Hair records a timeline as it grows. The segment near the scalp reflects the last few weeks, while the tips reflect older use. A sample cut today can show a pattern that stretches back months.
How Long Do Cannabis Metabolites Stay in Hair?
Most labs report a 90-day detection window for scalp hair, which matches about 1.5 inches of growth. Longer hair can extend that window if the lab tests older segments. Courts and employers often limit testing to the recent 90 days.
- About 7 days: earliest a new trace may appear near the root.
- About 30 days: 0.5 inch of new growth to test.
- About 90 days: standard window for pre-employment and legal tests.
- Past 90 days: possible with long hair and low cutoffs.
Can a Hair Test Show When You Used?
Segmental analysis cuts a strand into pieces and tests each piece. Every piece maps to a rough time window based on growth rate. The method gives a range, not a specific date.
What Does Not Change Hair Metabolite Levels?
Detox shampoos, vinegar rinses, and home remedies have little published proof behind them. Labs screen for tampering, and severe damage can void a sample. Water alone does not flush metabolite out of hair once it sits inside the strand.
How Do Labs Test Hair for Cannabis?
A sample goes through an immunoassay screen first. Positives get a second run on gas or liquid chromatography with mass spectrometry. The confirm step identifies THC-COOH at a set cutoff, often 1 pg/mg for screening and 0.1 pg/mg for confirmation.
Frequently Asked Questions
Does CBD show up as THC in a hair test?
CBD is a separate molecule and does not turn into THC in the body. Full-spectrum CBD can carry trace THC, and heavy use of those products may build measurable levels. A test that targets THC-COOH will not flag CBD on its own.
Can one use show up in hair?
Yes, though it is less common. A single low dose often falls below the cutoff. Higher doses or repeated use within the same month raise the odds of a positive.
Does hair dye remove metabolites?
It may lower the reading, not erase it. Labs look for signs of chemical damage and can reject a sample that shows tampering. No over-the-counter product has solid proof that it clears THC-COOH from hair.
Key Takeaways
- The main marker is THC-COOH, and the standard window is about 90 days.
- Dose, frequency, melanin, growth rate, body fat, and treatments all shift the result.
- External contamination is real, which is why labs wash samples.
- Detox shampoos and dyes do not clear metabolites and can flag tampering.