Cannabis in Hair: How Long It Stays Detectable (90 Days)

The short answer: cannabis stays detectable in hair for up to 90 days after the last use. That is the window most toxicology labs and clinical references cite for head hair, and it is the number to plan around. The math behind it is simple. Head hair grows about half an inch per month, and labs take a 1.5 inch segment from the scalp end, which covers roughly three months of history. This guide breaks the window down by hair type, cutoff level, and the factors that move it shorter or longer, so you can judge your own situation instead of leaning on one headline number.

How the 90 day window is calculated

A hair test is a record of growth, not of permanent storage. When THC and its metabolites travel through the blood, a small amount reaches the follicle and gets locked into the keratin as the strand forms. The strand keeps that record as it grows out. Head hair grows about 1.25 cm, or half an inch, per month. A lab takes roughly 3.8 cm, about 1.5 inches, from the scalp end and often cuts it into segments, each representing about a month of use. That produces a lookback of about 90 days and can show a pattern of use over that period rather than a simple yes or no.

Timing matters at the front end too. After use, it takes about one to two weeks for the drug to appear in the newly formed hair above the scalp. The 90 day clock starts at the last use, not at the test date, so a test taken a few days after a single session may miss that session.

Head hair versus body hair

Which hair is collected changes the answer, sometimes by months.

Head hair

Pros

• Standard sample for most labs, with a predictable 90 day lookback.
• Growth is steady, so segment analysis can map use over time.

Cons

• One heavy session can leave enough metabolite to register.
• Once the strand has grown, the record is fixed and cannot be cleared faster.

Body hair

Pros

• Slower growth and a longer resting phase can extend the detectable history past 90 days.
• Collected when head hair is too short to sample.

Cons

• The longer window works against anyone trying to show recent abstinence.
• Growth rates vary more between people, so results are harder to date.

What the lab is measuring

Most hair panels look for THC-COOH, the same metabolite urine tests target. The Society of Hair Testing suggests a cutoff of 0.1 picograms per milligram for THC-COOH in hair, an extremely small amount. Labs wash the sample before analysis to remove surface contamination from smoke, hands, or resin, then screen and confirm any positive, usually with mass spectrometry. Some newer panels also test for THC itself and for other cannabinoids. Routine tests do not separate hemp-derived CBD use from marijuana use with much confidence, and passive exposure is a known gray area that wash steps and cutoffs are designed to limit.

What lengthens or shortens the window

• Frequency and dose. Daily high-potency use builds a higher concentration in the strand than one session does.
• Metabolism. Faster clearance from blood can mean less metabolite reaching the follicle.
• Growth rate. People who grow hair faster than half an inch per month spread the same record across more length.
• Time and length. A fresh haircut removes old record only if the length is actually gone.
• Cutoff and method. A lower cutoff with a sensitive assay finds more positives at small concentrations.
• Cosmetic treatment. Bleaching, dyeing, perming, and relaxing damage the strand and can lower measurable levels in treated sections, but the effect is uneven.

What does not work

Detox shampoos, vinegar rinses, and hair cleanse kits are sold in volume and supported by little published evidence. They may strip surface residue that a lab would wash away anyway. They do not remove metabolite already built into the strand. Bleaching or repeated chemical treatment can reduce the signal in damaged sections, but it can also flag the sample as treated while leaving the rest of the strand intact. There is no product with consistent research backing that turns a positive hair test negative.

Hair test versus urine, blood, and saliva

Pros of hair testing

• A long window, roughly 90 days, that cannot be diluted with water.
• The sample is stable and easy to store for later confirmation.

Cons of hair testing

• It cannot show impairment or recent use, only exposure over weeks to months.
• It costs more than urine screening and falls outside the current federal mandatory workplace panel, so standards vary by lab and program.

Urine remains the default for most pre-employment screening and detects cannabis for about three days after occasional use and up to 30 days after heavy chronic use. Blood shows recent use over hours to a couple of days. Saliva covers roughly one to three days. Hair is the longest of the four.

Who the 90 day figure matters most for

Use the 90 day number if you face a court-ordered or child custody test, a safety-sensitive job program, or an employer that pays for hair analysis. Use a urine timeline if you are looking at a standard pre-employment screen, since that is the test most US employers run. If you use cannabis on a regular schedule and a hair test is coming, the only variables you control are time and length of hair. Nothing sold online reliably defeats the test, and the honest takeaway is that hair holds a record covering about three months, give or take a few weeks, depending on how fast your hair grows and where the lab sets its cutoff.