Three Things Every Employer Should Understand About Marijuana Impairment

Crisis Puzzle

By: Bill Current

The legalization of marijuana has resulted in more people using marijuana and more employees testing positive for marijuana. But it has not diminished the negative effects the drug can have on people who use it.

A myriad of studies, surveys and reports over a period of several decades indicate that marijuana can alter a user’s cognitive and psychomotor capabilities. In other words, the user can become impaired, from struggling to remember instructions to operating a motor vehicle. As such, employees who are under the influence of marijuana while on the job may not be able to properly perform their duties, which could affect the quality of their work and jeopardize the safety of the work environment.

Marijuana impairment is a subject of heated debate. Some pro-marijuana sources argue that marijuana is a relatively benign drug and that its effects only last about 20-30 minutes. This so-called “peak” window of impairment, after which impairment should no longer be of concern to employers, can be misleading. After all, someone who is impaired 4, 6 or 8 hours after using marijuana is, perhaps, just as dangerous to the workplace as someone who is impaired 30 minutes after usage.

For employers, the debate over impairment comes down to three critical issues:
1) What factors contribute to marijuana impairment?
2) How long does impairment last (the window of impairment)?
3) What drug testing method can detect marijuana during that window of impairment?

Contributing Factors to Marijuana Impairment

When considering the impairment issue, it is important to understand that intoxication and impairment are two different things. Referring to tetrahydrocannabinol (THC, the psychoactive ingredient in marijuana), a report from the American Automobile Association (AAA) stated: “THC can remain in a user’s blood or urine for weeks after they consume marijuana, depending on various factors. Furthermore, THC levels spike immediately after consumption, but decline to low levels very quickly – long before impairment ends.”

This means that even if a person no longer feels high from marijuana use, they could still be impaired, depending on several factors.

How marijuana is consumed or absorbed into the body, for example, affects impairment. One study found: “The length and intensity of intoxication [not impairment] depends not only on the strength of the marijuana product, but also on how the drug is consumed. Inhaling marijuana typically causes onset of intoxication within five minutes, with symptoms of intoxication [not impairment] lasting a couple of hours. On the other hand, ingesting marijuana (e.g., “special brownies”) can delay onset of intoxication between one to four hours, and intoxication can last much longer than that.”

A report by the National Highway Traffic Safety Administration (NHTSA) found the following: “Most behavioral and physiological effects [of marijuana use] return to baseline levels within 3-5 hours after drug use, although some investigators have demonstrated residual effects in specific behaviors up to 24 hours, such as complex divided attention tasks. Psychomotor impairment can persist after the perceived high has dissipated. In long-term users, even after periods of abstinence, selective attention (ability to filter out irrelevant information) has been shown to be adversely affected with increasing duration of use, and speed of information processing has been shown to be impaired with increasing frequency of use.”

From this we learn that marijuana-induced impairment is affected by how long ago the drug was used, whether the person is a “long-term user” and, among other factors, the tasks required.

Window of Impairment

A report from the University of Sydney claims to have accurately identified a window of impairment for cannabis users, “suggesting more accurate timeframes of intoxication from one of the drug’s active compounds, tetrahydrocannabinol (THC).” The research, published in Neuroscience & Biobehavioral Reviews, analyzed 80 separate studies to “determine when people would be impaired after cannabis use. Depending on how much THC is taken, how it’s taken, and the person taking it, they found cognitive impairment could last between three and ten hours.”

Again, we see a reference to multiple contributing factors to marijuana-induced impairment, including how much THC is consumed, how it was consumed, and the individual using the drug (e.g., a heavy user vs. an occasional user, etc.). How marijuana is used is particularly important. The same report found:

“Our analysis indicates that impairment may last up to 10 hours if high doses of THC are consumed orally. A more typical duration of impairment, however, is four hours, when lower doses of THC are consumed via smoking or vaporization and simpler tasks are undertaken (e.g., those using cognitive skills such as reaction time, sustained attention and working memory).”

“This impairment may extend up to six or seven hours if higher doses of THC are inhaled and complex tasks, such as driving, are assessed.”

And what about heavy users vs. occasional users? “Regular users of marijuana were found to experience less impairment than occasional users, but the authors also noted that regular users often took higher doses of THC and had less predictable levels of impairment after use.”

Drug Testing

Testing for marijuana is still legal in virtually every state with some conditions, New York state and Philadelphia, PA being exceptions. Some states do require employers to show evidence of impairment in addition to a positive test result before taking adverse employment action. However, no drug testing method can prove, either legally or scientifically, that a person is impaired. What most testing methods can prove is that someone has been using marijuana and, depending on the ingestion method, a range of time within which the drug is still present in that person’s system.

There are only two drug testing methodologies endorsed by both the federal government’s Substance Abuse and Mental Health Services Administration (SAMHSA) and the U.S. Department of Transportation (DOT): lab-based urine and lab-based oral fluid (pending final guidelines from DOT). There are rapid-result, point-of-collection testing (POCT) devices that have been cleared by the Food & Drug Administration (FDA), but SAMHSA has not yet issued guidelines for or authorized the use of any POCT devices.

The only federally endorsed drug test that is capable of revealing recent marijuana use is oral fluid testing. Because the parent drug and not just a metabolite of the drug can be detected in an oral fluid sample, detection is possible within 10-15 minutes after marijuana is used. The window of detection for THC extends for up to 24 hours, making oral fluid testing great to detect more recent use.

Urine and hair testing also accurately identify the presence of marijuana, but their respective windows of detection (3-4 days starting approximately 7 hours after usage for urine and up to 90 days starting 7-10 days after usage for hair testing) do not detect recent use and extend well beyond marijuana’s window of impairment.


Workplace drug testing has proven to be a powerful deterrent to drug use and an effective way of identifying people who need help. Testing applicants and employees for marijuana, especially those in safety-sensitive positions, is required under DOT and still permitted in virtually every state with some caveats. Further, employers still have the right to prohibit workers from using marijuana while on the job, bringing marijuana into the workplace, and being at work impaired by marijuana.

Understanding the employer’s objectives to putting in place or sustaining a drug testing program, i.e. company culture, safety concerns, can help employers choose a drug testing method that best aligns with their need to prohibit employees from being at work under the influence of marijuana while simultaneously complying with state laws that prohibit employers from discriminating against lawful marijuana users.


Comparison of Key Attributes of 5 Drug Testing Methods Relative to Testing for Marijuana
Urine Oral Fluid Hair Breath Blood
Lab-based or POCT Both Both Lab only POCT only Lab only
Window of Detection 3-4 days Up to 12 hours for THC / 24-48 hours for other drugs Up to 90 days 2-3 hours for marijuana; does not detect other drugs 2-3 days
Recent-Use Detection Lag time of several hours after initial usage Within several minutes after usage of any Lag time of 7-10 days after initial usage of any drug Within several minutes

marijuana usage only

Immediately after any drug usage
Legal by States Lab: 50 states

POCT: where permitted

Lab: 47 states

POCT: where permitted

Lab: Most states POCT: Only where POCT is permitted and where breath is permitted Usually not permitted for workplace testing
SAMHSA Regs Yes Yes Proposed No Only in very limited circumstances
Detection of Other Drugs Virtually all drugs Virtually all drugs Virtually all drugs Marijuana only Virtually all drugs
Alcohol Detection Yes Yes Yes Yes Yes

Current (2022).

National Survey on Drug Use and Health. SAMHSA. October 2021.  file:///C:/Users/bcurr/AppData/Local/Temp/Temp1_2020NSDUHDetTabs01112022%20(1).zip/NSDUHDetTabs1-1to1-5pe2020.pdf

Banta-Green and Williams (2016) Overview of Major Issues Regarding the Impacts of Alcohol and Marijuana on Driving Quest Diagnostics. 2022.

Children’s Hospital Colorado Acute Marijuana Intoxication

Wikipedia Psychomotor skills are defined as the “relationship between cognitive functions [thinking, remembering, understanding] and physical movement… demonstrated by physical skills such as movement, coordination, manipulation, dexterity, grace, strength, speed—actions which demonstrate the fine or gross motor skills, such as use of precision instruments or tools, and walking.”

Phiddian (2021) How long does impairment last after cannabis use?



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