Science Explains Just How Long Weed Stays in Your Hair

That moment of utter relaxation that accompanies peak stoned comes at a cost. Namely, after you’ve been asked to take a surprise drug test, and the 90 days of paranoia hits you harder than the hit did. Unfortunately, there’s not much you can do, except try not to reset the clock with another hit while you wait.

The most common method of drug testing involves taking a hair sample. Although controversial — research suggests it has a tendency to generate false positives — it’s the least invasive method to weed out drug users, making it appealing for employers.

Here’s how your luscious locks give you away. When you ingest weed, whether through smoking, vaping, or eating, the active compounds in weed, cannabinoids, take a stroll through your bloodstream. One of their stops includes the blood vessels that feed your scalp. From there, compounds leftover from metabolizing THC enter a strand of your hair from the base, at the hair papilla, waiting to emerge from your scalp for detection.

Drug tests usually analyze samples of hair close to the scalp, from the freshest 1.5 inches. Hair grows about 0.5 inches per month, which makes three months, or 90 days, the average amount of time before that 1.5-inch chunk is THC-free.

To test a sample, hair is first washed to remove contaminants and then chopped up for digestion, where chemicals break it down into keratin and any other substances that may be present. The sample is run through ELISA assay (enzyme-linked immunosorbent assay) that uses antibodies that bind with metabolites of THC. If this result comes back positive, scientists may use analytical chemistry methods like gas chromatography or mass spectroscopy to confirm exactly what substances are in your hair.

If the final concentration of THC metabolites in your sample surpasses 0.1 picograms per milligram, it’s pretty certain that you’ve used marijuana in the past three months, according to Quest Diagnostics, one of the premier drug-testing labs in the United States.

Even after three months, those 1.5 inches are a dangling record of that time you lit up — although the concentration decreases with time, the compounds may still remain. So if you’re truly desperate for a quick fix to get rid of the evidence? Shave your head.

Correction: A previous version of the story printed an erroneous statement that THC compounds remain in hair indefinitely. It has been updated with additional comment from the study’s author.