You smoked with your friends last weekend, but in the cold harsh light of Monday, you might be having regrets — especially if your employer conducts drug hair tests. From the moment you get high, it takes about 90 days before you’re free and clear when it comes to the THC in your hair. But exactly how long does weed stay in your hair? Here’s what you need to know.

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The dreaded hair test is one of the most used tests for finding drug users. It’s less invasive and takes less time than urine and blood tests, but it’s also controversial and has been known to cause false positives — even coming in contact with smoke can be enough to mess up your results. But until this issue is sorted out, it’s worth knowing exactly how hair tests work, especially for one of the most common drugs around: weed.

How does weed get in my hair, Anyway?

When you smoke, vape, or eat marijuana, all you’re really doing is facilitating the release of the drug’s active compounds into your bloodstream. Once they’re in your veins, these cannabinoids ride the blood highway to the cannabinoid receptors on the body’s cells, where they bind and exert their effects. Along the way, some of the cannabinoids — metabolites of psychoactive THC are what most tests look for — enter the blood vessels that feed the cells of the scalp. There, at the interface between the vessel and the follicle cells known as the hair papilla, THC jumps the gap, entering the matrix, where actual hair growth occurs. Over the next couple of weeks, the hair from the root sprouts upward, carrying THC past the scalp, where it becomes fair game for testing for the next three months.


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Active compounds from marijuana travel in the bloodstream to a strand of hair via the hair papilla in the hair follicle.Wikimedia

How long does weed stay in your hair?

The most commonly cited number for the amount of time marijuana can be detected in your hair is 90 days. That time frame is calculated on the basis of how long it takes for hair at the root to grow past the scalp, and it takes into account the part of the hair that drug testers sample.

According to Quest Diagnostics, one of the premier drug testing labs in the United States, it takes five to 10 days before hair at the root pushes out past the scalp and into the world. In a drug test, samples are usually taken from hair 1.5 inches from the scalp. Since hair grows at about 0.5 inches per month, then you have about 3 months, or 90 days, before you’re in the clear.

How does hair testing detect THC?


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ELISA assays use specific antibodies to look for marijuana compounds in a sample of digested hair.Flickr / JuVlai

Once your hair is collected, it’s washed to remove any external contaminants, chopped up, and then digested in a solution meant to break it down into its components — largely the protein keratin, together with any other compounds that make their way into the hair shaft. Usually, that solution is put through an ELISA (enzyme-linked immunosorbent assay) screen, a laboratory procedure that uses specific antibodies to bind the target molecule of choice — in this case, THC. (More specifically, THCA, a direct metabolite of THC.)

In the case of a positive test, another sample of the same hair is usually put through a secondary test that uses techniques called gas chromatography and/or mass spectrometry. These techniques, known as GC/MS and GC/MS/MS (tandem MS), are more accurate, more involved techniques that scan the sample for the “molecular signature” of the target compound.

The final concentration of the chemical in a sample of hair gives the final verdict. The “confirmatory level” of THCA that Quest Diagnostics uses is 0.1 picograms per milligram of hair. Anything above that pretty much confirms you’ve used marijuana in the past three months.

What kind of hair testing errors are possible?

There are a lot of variables in hair testing for marijuana that make it a very controversial way to decide whether or not a person is fit to have a job. As mentioned above, false positives are very possible. It’s still not entirely clear, for example, whether hair can incorporate marijuana-derived compounds after simply being in the same vicinity as marijuana smoke.

Hair color — that is, hair melanin content — seems to alter the sensitivity of the test, with darker-colored hair being more sensitive to the test, according to a report from the Substance Abuse and Mental Health Services Administration. There has also been some concern about gray hair frequently turning up false positives, though more research is needed to explain why this happens.

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Then, of course, not everyone’s hair grows at the same rate, so it’s possible that your hair may rat you out for drug use far past the 90-day date if your hair grows slowly. In any case, the compound doesn’t leave your hair after that date — it simply stays in the hair as it keeps growing longer — so the only way to truly get rid of all traces of marijuana is to cut if off altogether.