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Cannabis impairment is difficult to determine using police field sobriety tests

As society continues to grapple with the implications of wider cannabis legalization, one major concern that has emerged is the impact on road safety from cannabis impairment.

Research has proven that cannabis impairs essential driving skills such as reaction time, decision-making, coordination, and perception. California, for example, has seen a shocking 62% rise in fatal accidents involving drug-related impairment over the past three years.

Determining cannabis impairment, however, is no easy feat. Unlike the clear correlation between blood alcohol concentrations and impairment, there’s no such association with the blood concentration of tetrahydrocannabinol (THC), the psychoactive compound found in cannabis.

Instead, officers have relied on behavioral assessments or field sobriety tests to gauge the level of impairment. Yet, these tests were predominantly validated based on alcohol ingestion, leaving doubts about their effectiveness in identifying cannabis impairment.

How to study cannabis impairment

Seeking to bring clarity to this issue, researchers at the University of California San Diego Center for Medicinal Cannabis Research embarked on a double-blind, placebo-controlled randomized clinical trial.

Published in JAMA Psychiatry on August 2, 2023, the study sought to assess the accuracy of field sobriety tests in detecting drivers under the influence of THC.

The results indicated that officers could, at specific time points, distinguish between individuals who had consumed THC and those who had not based on the tests. However, the overall reliability of these tests may not be enough to definitively signal THC impairment.

“Driving is a complex task that requires intact attention and motor skills to stay safe,” said lead author Thomas Marcotte, PhD, a professor of psychiatry at UC San Diego School of Medicine and co-director of the Center for Medicinal Cannabis Research.

Marcotte noted the individual variability in the impact of cannabis. He also highlighted the public health necessity to ensure effective and unbiased evaluations of impairment. “This study is an important step towards that goal.”

How researchers conducted the study

Involving 184 adult cannabis users aged between 21 and 55, the study used a placebo-controlled model. 63 participants received a placebo cannabis cigarette and 121 received a THC-infused cannabis cigarette.

After consumption, participants reported a median highness level of 64 on a scale of 0 to 100, indicating significant intoxication.

Following this, law enforcement officers, well-trained in field sobriety tests, assessed the participants’ abilities such as balance, coordination, divided attention, and eye movements.

These tests, including the Walk and Turn, One Leg Stand, Finger to Nose, Lack of Convergence, and Modified Romberg tests, were conducted at four different time intervals post-smoking.

What the scientists learned

According to the results, officers classified a significantly higher proportion of THC group participants as impaired based on the field sobriety tests at three of the four time points measured.

For instance, one hour after smoking, 81% of participants from the THC group were identified as impaired. This was compared to 49% incorrectly identified from the placebo group.

Despite this, officers suspected that almost all participants who failed the tests had consumed THC. This occurred regardless of whether they were in the THC or placebo group.

While participants also completed a driving simulation correlated with the results of select field sobriety tests, officers weren’t privy to this information. These findings suggest that while existing field sobriety tests might be sensitive enough to detect cannabis influence, they aren’t effective.

Cannabis impairment testing needs a new method

The high overlap in poor test performance between the THC and placebo groups indicates these tests alone may not be sufficient for identifying THC-specific driving impairment.

Marcotte reiterated that while “field sobriety tests are useful additions to overall evaluations of drivers, they are not accurate enough on their own to determine THC impairment.” He called for the development of new, effective measures to identify cannabis impairment to ensure road safety.

Recognizing the importance of this issue, the UC San Diego Center for Medicinal Cannabis Research has teamed up with the California Department of Motor Vehicles and the California Highway Patrol to launch a follow-up study.

This study, aiming to test various methods for detecting cannabis-impaired driving, will recruit 300 participants and is slated to commence in late summer 2023.

More about cannabis impairment

Cannabis impairment refers to the physical and cognitive effects that occur after consuming the psychoactive compound found in cannabis. This is known as tetrahydrocannabinol (THC).

The extent and nature of impairment can vary widely among individuals. Factors such as dosage, frequency of use, tolerance, and individual physiology influence it. Common effects include altered perception, impaired coordination, slowed reaction times, and memory loss.

Effects on cognitive and physical functions

The most prominent impact of cannabis use is on cognitive and physical functions. THC binds to cannabinoid receptors in the brain, affecting areas that regulate memory, concentration, and motor functions.

Cannabis users often experience difficulty with short-term memory, decreased problem-solving abilities, impaired motor coordination, and slowed reaction time. These effects pose significant challenges to tasks that require intact attention and motor skills, such as driving.

Duration and intensity of impairment

The duration and intensity of cannabis impairment can vary greatly depending on several factors. These include the amount of THC consumed, the method of consumption, and the user’s tolerance to the substance.

Impairment can last for several hours after consumption, and residual effects may persist for several days with high doses. With chronic use, some degree of impairment may become persistent. This can potentially affect cognitive functioning even when the user is not acutely intoxicated.

Detection of cannabis impairment

Detecting cannabis impairment, particularly in a driving context, remains a complex challenge. Unlike alcohol, there is no direct correlation between blood THC concentration and impairment level.

Standard field sobriety tests, developed primarily to detect alcohol-induced impairment, have limited effectiveness in accurately identifying cannabis-impaired drivers. Researchers are currently exploring potential biological markers and psychomotor tests to improve detection.

Cannabis and driving impairment

Cannabis impairment poses significant risks to road safety. Impaired motor skills, slowed reaction time, and altered perception can greatly increase the likelihood of traffic accidents.

Some studies have indicated a twofold increase in the risk of a collision following cannabis use. However, identifying cannabis-impaired drivers is challenging due to the lack of a reliable field test like a breathalyzer used for alcohol detection.

Legal implications

Legal implications for cannabis impairment, especially for driving under the influence, are evolving alongside the trend of cannabis legalization.

Many jurisdictions struggle with establishing a legal limit for THC akin to the blood alcohol concentration limits for alcohol. This is due to the lack of a direct correlation between blood THC concentration and impairment.

Penalties for driving under the influence of cannabis vary by location, but they can be severe, including fines, license suspension, and imprisonment.

Future research

Significant research is ongoing to better understand cannabis impairment, develop reliable detection methods, and mitigate its impact on public safety. Efforts include studies on how THC affects different individuals, examination of long-term effects, and development of potential countermeasures.

A key area of focus is the development of more accurate field sobriety tests and other assessment tools to effectively identify cannabis-impaired drivers.

Cannabis impairment is a significant concern for public safety, particularly on the roads. As the legal status of cannabis continues to evolve, so too does the need for effective measures to detect impairment and mitigate the risks associated with its use. The complexity of this issue demands continued research and policy development.

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