How Accurate is Hair Testing for Marijuana Use?

The upcoming legalization of cannabis in October has Canadian employers looking for various ways to test for marijuana and cannabis use in the workplace. This has lead to increasing interest in hair testing for marijuana use.

How accurate is hair testing for marijuana use compared to other methods? We’ll examine this question below.

Hair Testing for Marijuana Use: How it Works

Most employers are familiar with urine and saliva tests, which analyze bodily fluids to determine if drugs are present in the test subject’s body. Hair testing, in contrast, flags the presence of drugs in a hair follicle sample. Hair can be used to test for numerous substances, including prescription drugs and cannabis.

THC and other cannabinoids are deposited in hair follicles through the user’s bloodstream, binding to the tissue and cells surrounding the root of the hair. The hair containing the drug peaks above the scalp between 5 and 7 days later.

Unlike cannabinoids in blood, traces of cannabis remain in the follicle long after the person has used the drug, meaning it can be detected weeks or months afterwards. Once THC is present in the hair, there is no way to remove it. However, when the hair is cut off, the new hair that grows in its place will not contain THC unless the person has used within the 5-7-day timeframe.

Accuracy of a Hair Test for Marijuana and Cannabis Use

Hair testing is recognized as being a highly accurate method of testing for drug use. It takes just one picogram per milligram of THC to trigger a positive result in a hair test for marijuana use and requires only 1.5 inches of hair to test (an amount comparable to the tip of a shoelace.) It is said to be 6-10 times more effective than a urine test.

Hair Testing vs. Other Tests for Marijuana and Cannabis Use

  • Compared to urine testing: cannabis is detectable in urine up to 72 hours after use. A hair test can detect cannabis use over a period of months. Since a hair sample can be collected from the test subject directly (whereas a urine sample is given in private), it is impossible to ‘beat’ a hair test by surreptitiously swapping in a ‘clean’ sample from someone else.
  • Compared to blood testing: blood tests can detect cannabis for a maximum of 7 days after use, while hair testing can detect cannabis months after use. Hair tests for marijuana and cannabis use are less invasive to collect than blood tests, which are administered by technicians in laboratory environment.