When you toke up on 4/20, you’re living for the moment. Forget today’s stress; ignore yesterday’s regrets; remain blissfully unaware of what fresh hell tomorrow may bring. But all that changes on 4/21, which marks the first of about 90 days during which you will almost definitely fail a drug hair test. For anyone on the market for a new job, the near-to-middle future is about to be very bad. There isn’t much you can do about it.

The hair test is one of the most commonly used tests for identifying drug users, largely because it’s less invasive and time-intensive than urine and blood tests. Its widespread use has been quite controversial, as studies have shown that it tends to turn up a lot of false positives, sometimes suggesting that a person has used marijuana when they were actually just exposed to smoke externally. While policymakers decide whether it’s a fair way to assess drug use, however, it doesn’t hurt to know how it actually works, when it works.

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.

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

How Long Does It Stay There?

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 Compounds?

ELISA-test
ELISA assays use specific antibodies to look for marijuana compounds in a sample of digested hair.

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 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.

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.