Hair Follicle Test THC Levels: Cutoffs, pg/mg Numbers and What a Weed Result Really Means

Updated 21 Oct 2026 · By the DrugTestAnswers Editorial Team

Weed is the drug people worry about most on a hair test, and it's also the one with the strangest numbers. The cutoffs are so small that they look like typos, the compound the lab hunts for isn't the one you smoked, and the same habit can show up positive for one person and negative for the next. If you're staring at a report, trying to guess where your own use might land, or just want to know what "THC levels" even means for hair, this page lays it out the way a friend who understands lab work would explain it. No scare tactics, no promises, just how the numbers work and what you can actually do with them. We may earn a commission if you buy through links on this page.

The quick version

A hair test for marijuana doesn't measure how high you got or how much you smoked last weekend. It measures a tiny amount of a THC breakdown product, called THC-COOH, locked inside the hair shaft. Labs report it in picograms per milligram of hair (pg/mg), and the line between negative and positive is usually set somewhere between a few hundredths of a picogram and about one picogram per milligram, depending on the lab. Those are some of the lowest cutoffs used for any drug, because very little THC-COOH ever reaches hair in the first place.

Three things follow from that. First, regular users are much more likely to test positive than occasional users. Second, nobody can read a pg/mg number and tell you how many joints a week someone smoked. Third, the only reliable way to keep weed off a hair result is not using during the window the sample covers, which for a standard head hair sample is roughly the last three months.

THC, THC-COOH and why the lab cares about the difference

When you smoke, vape or eat cannabis, the main active compound is delta-9 THC. Your liver breaks it down into several products, and the one labs focus on is 11-nor-9-carboxy-THC, written as THC-COOH or carboxy-THC. It doesn't get you high. It's simply evidence that THC passed through your body.

Hair labs can test for both, but they treat them differently:

• THC (the parent compound) can land on the outside of hair from smoke in the air, from hands, or from a pipe or bong sitting nearby. Finding THC alone doesn't prove someone used it.
• THC-COOH (the metabolite) is made inside the body. It isn't in cannabis smoke to any meaningful degree, so finding it in washed hair is the stronger sign of actual use.
• CBN and CBD are other cannabis compounds that some labs measure for research or forensic cases. Most employment tests don't report them.

That's why most workplace hair tests that confirm a marijuana positive do it on THC-COOH. When people search for "THC levels in hair," the number they usually mean is really a THC-COOH number.

How small these numbers really are

A picogram is a trillionth of a gram. A typical hair sample weighs around 100 milligrams, roughly a pencil-thick lock cut close to the scalp. So a result of 1 pg/mg means about 100 trillionths of a gram of THC-COOH in the entire sample. A result of 0.05 pg/mg means a twentieth of that.

Compare that with cocaine, where the usual cutoff is 500 pg/mg, or amphetamines at 300 to 500 pg/mg. The marijuana metabolite cutoff can be thousands of times lower. If you want to see how other drugs compare, our guide to cocaine levels in hair walks through a drug with much higher numbers.

The cutoffs labs actually use

No single law sets hair cutoffs for private employers in the U.S. Each lab validates its own limits, and most of them sit close to two published references:

The federal guidelines are still only a proposal, and they don't cover private employers anyway, but labs use them as a benchmark. Your report should list the exact cutoffs that lab used, and those are the only ones that matter for your result. For urine and mouth swab numbers side by side, see our drug test cutoff levels table.

Screening, then confirmation

Every hair test runs in two stages. The screen is an immunoassay, a fast chemical test that reacts to THC-COOH and similar compounds as a group. If the sample comes in under the screening cutoff, it's reported negative and that's usually the end of it.

If the screen comes in at or above the cutoff, the lab moves to confirmation with mass spectrometry, usually GC-MS/MS or LC-MS/MS. Because THC-COOH levels are so low, labs need very sensitive equipment for this step. Confirmation measures THC-COOH by itself, and only a confirmed result at or above the cutoff counts as positive. A sample can fail the screen and still come back negative at confirmation. Our explainer on screening vs confirmation results covers this two-step process for every test type.

Is there a THC levels chart for light, moderate and heavy use?

This is the question behind most searches, and the honest answer is no, not one you can trust for an individual result. You'll see charts online that put ranges like "0.1 to 1 pg/mg = light use" and "over 5 pg/mg = heavy use." Treat those as rough illustrations at best.

Research that compares people's reported use with their hair results finds the same pattern again and again. Daily users test positive far more often than occasional users, and their numbers tend to run higher on average. But the spread inside each group is huge. Two people who smoke the same amount can have results several times apart, and some daily users still come back under the cutoff. A number on a report can tell a lab that use happened. It can't tell anyone how much, how often or exactly when.

Why the spread is so wide:

• THC-COOH is acidic. Most drugs that show up strongly in hair are basic compounds that bind to melanin, the pigment in hair. THC-COOH binds poorly, so only a small and variable fraction makes it in.
• People process THC differently. Body fat, liver function, genetics and tolerance all change how much THC-COOH circulates.
• Hair itself varies. Growth rate, thickness, oiliness, and cosmetic treatments all shift what ends up in the sample.
• Method of use matters. Smoking, vaping and edibles produce different blood levels over different time spans.

A rough picture of where use patterns tend to land

With that warning in mind, this is the general shape research and lab experience point to. It isn't a chart for your result, and nobody should use it to predict a pass.

For a closer look at the light-use end, our guide to hair tests and occasional use goes deeper.

How the sample length turns into a time window

Head hair grows about half an inch, or a bit over a centimeter, per month on average. Most labs cut the 1.5 inches closest to the scalp, which covers roughly the last 90 days. A couple of details matter for THC levels in particular:

• The newest week or so isn't in the sample. Hair takes several days to grow out past the skin, so very recent use may not show yet.
• The number is an average across the segment. A single result covers the whole 1.5 inches. Heavy use for one month and none for two can produce a result similar to steady light use for three months.
• Longer hair doesn't stretch the window by default. The lab uses the first 1.5 inches unless the employer orders more.
• Body hair is different. Arm, leg or chest hair grows more slowly and in cycles, so it can reflect a longer, fuzzier time span.

To see how timing works across drugs, read our guide to how long it takes to pass a hair follicle test.

Segmental testing: when the lab cuts the hair into months

Some reports split the hair into pieces, often three half-inch segments, and give a THC-COOH number for each. That's more common in forensic, family court and treatment cases than in hiring. A segmental report can show a pattern, like a positive in the oldest segment and negatives in the newer ones, which suggests use stopped. Even then, labs warn that hair growth isn't perfectly even and that compounds can shift a little along the shaft, so the month boundaries are approximate.

The wash step and secondhand smoke

Before testing, labs wash the hair with solvents to strip off surface contamination. Some keep the wash liquid and test it too. If there's a lot of THC in the wash and little or no THC-COOH in the hair, that points toward outside contamination rather than use.

That's the main reason secondhand smoke alone rarely causes a confirmed positive at labs that confirm on THC-COOH. Smoke can coat your hair with THC, but it doesn't put meaningful THC-COOH inside it. One caveat worth knowing: research has shown that THC-COOH can move onto someone's hair through close contact with a user's sweat or hands, which is part of why washing and cutoffs matter. Our page on secondhand smoke and hair tests covers this in more detail.

Smoking vs vaping vs edibles

All three put THC into your bloodstream, so all three can produce THC-COOH in hair. Edibles skip the smoke, which means less chance of THC landing on the outside of your hair, but the THC-COOH that matters still forms inside your body. Concentrates and high-potency vapes deliver more THC per session, which can push levels higher for the same number of sessions. Switching methods doesn't change the basic picture.

Delta-8, delta-9 and CBD

Delta-8 THC breaks down into its own carboxy metabolite, which is a slightly different molecule from delta-9 THC-COOH. Many screening tests react to both. Whether confirmation tells them apart depends on the lab's method, and many workplace reports simply say marijuana metabolite positive or negative. Don't count on delta-8 reading as "clean."

Pure CBD shouldn't produce THC-COOH. The risk comes from full-spectrum products, which can legally carry small amounts of delta-9 THC, and from products that are mislabeled. Regular heavy use of a full-spectrum product can add up. Our guide to delta-8 vs delta-9 drug test results explains the details.

What pushes a THC level up or down

Reading a THC line on your report

Workplace reports are usually simple. You'll see the drug class, the result, and sometimes the cutoff. An example, using made-up numbers just to show the layout:

Some employers only see "positive" or "negative" for marijuana, not the number. If a number is listed, it's the confirmation value that decides the result. Before an employer sees a non-negative result, a medical review officer (MRO), a doctor who reviews lab results, may contact you to ask about prescriptions or other explanations. For marijuana, a state medical card doesn't change the lab result, though some states limit how employers can use it. For a full walkthrough of report sections, see our guide to hair strand test results.

Check your own hair before your test

If you're worried, the most useful thing you can do is find out where you stand privately. A home hair test kit lets you snip a small sample, mail it to a lab, and get a result before an employer ever sees one. Lots of people who expect to fail actually pass, especially light users, and some who expect to pass are surprised. Either way, you'll be making decisions with real information. Our guide to home hair follicle test kits explains how they work.

Can a shampoo lower THC levels in hair?

This is where a lot of money gets spent, so here are the plain facts. TestClear sells Old Style Aloe Toxin Rid, a deep-cleansing shampoo aimed at people facing hair tests. TestClear says it's designed to clean the hair shaft and lists directions that involve repeated washes over several days before a test. What TestClear claims and what's been proven are different things. There's no independent published study showing it pushes THC-COOH inside the hair below a lab cutoff, so nobody can honestly say it works or promise a pass. Our Aloe Toxin Rid review and maker's directions separates the product facts from the hype.

Keep in mind what you're up against. Labs wash the hair before testing precisely to remove what's on the surface, and THC-COOH sits inside the hair. That's why we suggest a home hair test before you buy anything, and why the only sure way to pass is not using during the window the test covers.

About the Macujo and Jerry G methods

You'll also run into the Macujo method and the Jerry G method on forums. Both are home routines that combine strong cleansing shampoos with household products. The Macujo method typically uses vinegar, salicylic acid acne wash, and detergent shampoo in repeated rounds. The Jerry G method involves bleach and dye. Neither has independent evidence behind it, and both can damage your scalp and hair. If you try anything, never mix bleach with vinegar, acids, ammonia or other cleaners, because the fumes can be dangerous. Stop if your scalp burns or blisters. Badly damaged hair can also draw attention at collection. We cover these factually in does the Macujo method work for THC.

What actually changes a THC number over time

• Stopping use. New hair grows in without THC-COOH. After about 90 days of no use, a standard head hair sample is made entirely of hair grown since you stopped.
• Cutting back. Using less often lowers the amount reaching new hair, though it won't erase what's already there.
• Time and growth. Every month, about half an inch of older hair moves out of the tested segment.

Shaving your head doesn't make the problem go away, because collectors can take body hair instead, and body hair can cover a longer span.

Myths about THC levels in hair

• "One hit will show up for 90 days." Usually not. A single use often doesn't reach the cutoff, though it isn't impossible.
• "A higher number means you're impaired." No. Hair says nothing about impairment. It reflects past use over weeks or months.
• "Dark hair always tests higher for weed." Color matters much less for THC-COOH than for many other drugs.
• "Edibles don't show on a hair test." They can. The metabolite forms whether you smoke, vape or eat it.
• "Secondhand smoke will make you fail." Rare at labs that wash the sample and confirm on THC-COOH.
• "There's a chart that turns pg/mg into grams per week." Not one that holds up for an individual person.

Tests this page doesn't apply to

Nothing on this page applies to DOT, CDL, FAA, federal, military, court, probation or parole testing. Don't try to alter, dilute or avoid those tests. Tampering can carry legal consequences, and in DOT-regulated jobs it's treated the same as a refusal.

Where this information comes from

The cutoffs and lab steps here come from the HHS proposed Mandatory Guidelines for Federal Workplace Drug Testing Programs using hair (Federal Register, 2020), published Society of Hair Testing recommendations, and peer-reviewed research on cannabinoids in hair. Individual labs set their own validated limits, so always go by the cutoffs printed on your report.

If cutting back has been hard

If a test is making you rethink how much you use, that's worth listening to. Plenty of people find it harder to stop than they expected, and talking to someone confidential can help without judgment.

Disclaimer and support

This page is general information, not medical or legal advice, and lab methods change. Results depend on the lab, the sample and the person. For free, confidential help with substance use 24/7, call the SAMHSA National Helpline at 1-800-662-4357. If you're in crisis, call or text 988.