Pro-tip: if you are attempting to pass a drug test that involves a hair sample, you may want to skip a haircut like the one sported by this reveler at the 2013 Cannabis Cup in Denver. (Seth A. McConnell, Denver Post file)

Hair drug testing: Useful information for marijuana consumers

Ask the Cannabist: Wondering how to pass a hair marijuana drug test? Experts shed light on what your hair reveals

We recently received two questions from readers wondering about the same thing: hair drug testing. Here are their situations:

Hey, Cannabist!
I smoked daily until six months ago, then approximately two weeks off, two weeks on. I quit totally just over two months ago. I had to go for a drug test (hair) a couple days ago and I am waiting for the results. What are your thoughts about me passing the THC test? I am very concerned!
–Tainted Tresses

Hey, Cannabist!
My son is trying to get a job and they have a no-drugs policy in force. He was a heavy medical marijuana user for years. He quit smoking about 115 days ago. He has taken two hair follicle tests approximately three weeks apart. The first test, approximately three weeks ago, said his level was .49. When his first test came back at .49 he shaved his beard again where they had taken the sample from. His second test just came back at .14. He’s wondering if he should shave again? (He shaves his entire body.) Would you have any idea with his current level at .14 how long would it take to get to level .1? He has a job, he just needs a negative drug test. Any help will be appreciated!!!
–Mammaduke Marihoochie Do 

Hey, Tresses and Mammaduke!

Let’s brush up on our hair samples and drug testing knowledge.

I asked an expert with Quest Diagnostics, one of the larger drug-testing companies in the United States, for details on how hair drug tests work.  

“Unlike urine drug testing, which may only detect drug use within the past 2-3 days, hair testing is able to detect a pattern of repetitive drug use for up to 90 days,” said Barry Sample, Quest director of science and technology, via email.

How does he come up with this number of days?

An overview of hair drug testing can be found on the Quest Diagnostics Hair Testing FAQ page. Hair samples are used because drugs in the bloodstream bind to hair follicles underneath the scalp. It takes five to 10 days for the drug-containing hair to reach the surface of the scalp. Head hair grows an average of half an inch per month. The standard hair sample is 1.5 inches from the scalp. 1.5 divided by half an inch means about three months or 90 days.

The hair sample is prepped and the drug test detects as little as 1 picogram per milligram (pg/mg) of THC carboxylic acid metabolite. If the sample tests positive, it goes through a second test to confirm the results.

Individual variables

Now that we know the basics, let’s move on to specifics for marijuana drug testing. Based on Sample’s statement, ideally, for your hair and you to pass a marijuana drug test, you need to abstain from consumption for at least 90 days. This is definitely not a guarantee, but a good starting point.

So, Tresses, you probably need more elapsed time before your hair samples do not indicate positive for marijuana use. Since you’ve been abstaining for 60 days, it is possible some of your repetitive marijuana use will be detected in a test. Even if you consumed for two weeks on and abstained for two weeks, the repetitive consumption is within the 90-day detection period. Your individual variation of rate of hair growth may be faster or slower than the average of a half-inch per month. This difference could affect the results.

Speaking of variation, what about Mammaduke’s son? It sounds like he’s been abstaining from smoking marijuana, as well as shaving his body hair for almost four months, in order to pass a pre-employment drug screening.

First of all, Mammaduke, it’s sweet you are writing for your son! Let’s get more info from the experts about your son’s situation. Maybe he won’t feel compelled to shave again, unless he’s into that sort of thing.

Sample said: “As with all drug tests, the window of detection is dependent on the specific drug being used, the pattern of use, the strength or dose of the drug and the individual’s metabolism.”

Since your son has passed up the puffing and passing for more than 90 days now but is still testing positive, it’s likely these types of individual variables are factors. The pattern of usage such as frequency and the potency of the medical marijuana consumed can affect the results. Additionally, your son’s metabolism affects how quickly his body metabolizes the THC molecules.

Hair types can make a difference

Other variables worth mentioning that can affect testing accuracy is the interference in the test results due to some types of hair and exposure to secondhand smoke.

A 2013 presentation, available at the federal Substance Abuse and Mental Health Services Administration (SAMHSA) website, shows the standard method of collecting hair samples, how the samples are prepared to remove external contamination and the instruments used to analyze the samples. Even with the rigors of lab methods used on hair samples for drug analysis, the report describes interference problems with some hair types. Gray hair for example, can result in false positives.

For more on interference, I turned to Paul Armentano, deputy director of the National Organization for the Reform of Marijuana Laws (NORML) and 2013 recipient of the Alfred R. Lindesmith Award for Achievement in the Field of Scholarship, for his encyclopedic knowledge of marijuana and health studies.

“The sensitivity of this test depends on various factors, including hair color — the test is more sensitive for those with darker hair — and the type of drug that is being detected,” Armentano said via email. “The test is highly sensitive to cocaine and less sensitive to occasional marijuana use.”

He referred to a study titled “Cannabinoid concentrations in hair from documented cannabis users” and pointed out the finding that “detection rates were significantly different … between daily cannabis users (85%) and non-daily users (52%).”

Armentano added: “Environmental exposure that may cause damage to the hair (bleaching, chemical stripping, etc.) is presumed to impact the test’s sensitivity, but I’m not aware of any recent clinical data on the subject.”

Thus if your son has darker hair, the THC detection in the analysis can be more sensitive than with lighter hair shades. Also, chemical processing and sun exposure could perhaps affect the validity of the results. Although some factors that create false-positive results are not relevant for your son, the interference mentioned shows the known limitations to accuracy in hair testing for THC.

That said, depending on the reporting threshold for THC in the test your son has to take, his current results may already be lower than the level that indicates a positive test. As mentioned above, Quest Diagnostics tests detect down to 1 picogram per milligram (pg/mg) of THC carboxylic acid metabolite. If your son’s level is .14, (I am assuming you are referring to pg/mg), he is likely now below the point that triggers a positive result.  

Unless you know the reporting threshold, we can’t confirm his current results are low enough to be inconsequential, so your son should probably wait a few more weeks for good measure before taking the employment drug test. With his physiological variables and previous medical marijuana consumption habits, it’s taking longer for him to pass this test to start a new chapter in his life. XO


Ask The Cannabist: Clearly you have questions about marijuana, be it a legal concern, a health curiosity or something more far-reaching. Email your queries to askthecannabist@gmail.com.