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Drug Tests - Methods of Detecting Cannabis Use

Some medical cannabis users have encountered drug testing for employment, school, traffic incidents, legal requirements, etc. In some cases, having medical approval for marijuana is accepted, in others, it is not.

While some tests seek to determine if the "parent drug" is in the system (such as THC), others test for drug metabolites 1, which are produced by the body's chemistry after a drug is ingested. This can present problems for some patients who test positive for THC metabolites, yet they may not have used marijuana for several weeks.

When medical use is not accepted by authorities, some patients may be vulnerable to arrest and sanctions depending upon the outcome of these drug tests. The following chart examines the four testing methods currently in use, and what that may mean to medical marijuana patients.
Hair cuticle

Hair 2  3
Tests for parent drugs, or for drug metabolites? 1 Metabolites Both Parent Drug Metabolites
Length of time marijuana can be detected by this type of test Between a few days and several weeks Usually a few hours after past use, sometimes 1-2 days Usually a few hours after past use, sometimes 1-2 days Approx. 90 days after past use
Length of time needed after use before can be detected in test 2-8 hours Immediately Immediately 5-7 days
Currently used in "driving under the influence" tests? Yes Yes Not in U.S., used in some European countries. No
Currently used in employee or pre-employment tests? Yes Yes Yes Yes
Currently used in student testing? Yes Yes Yes Yes
Permitted as court evidence? Yes Yes Yes Yes

1 The term "parent drug" refers to the identifiable psychoactive compound of a controlled substance. For cannabis-based drugs, this is known as THC. Drug metabolites refer to those substances produced by the metabolism (body's chemistry) after a drug is ingested. Though the presence of metabolites in blood or urine is indicative that a certain drug may have been previously consumed, not all metabolites are psychoactive. Marijuana's metabolite, THC-COOH, is not considered psychoactive. Detection of this metabolite does not indicate or prove that the parent drug is still in they body.

The U.S. Department of Justice states that a positive drug test result for the presence of a drug metabolite "does not indicate... recency, frequency, or amount of use; or impairment." 12/92 U.S. DOJ, Bureau of Justice Statistics.

The U.S. Department of Transportation states that while a positive test for drug metabolites is "solid proof of drug use within the last few days, it cannot be used by itself to prove behavioral impairment during a focal event."

2 Hair testing detects "drug molecules permanently entrapped in the hair following ingestion."
3 "[S]tudies have found dark-haired people are more likely to test positive for drugs because they have higher levels of melanin, which allows drug compounds to bind more easily to their hair."
8/24/05 Associated Press

Thanks to Paul Armentano, Sr. Policy Analyst, NORML, and his report "You Are Going Directly to Jail" (PDF 235 KB) and to Psychemedics Corporation, for information compiled for this chart.