A two-year study found that at least 50% of subjects who smoked cannabis with THC in a randomized clinical trial had a significantly diminished ability to drive compared to subjects who took a placebo cigarette. This study was conducted at the UC San Diego Center for Medical Cannabis Research (CMCR) and was published in the Jan. 26 online issue of JAMA Psychiatry. 

Before this study, there were a number of small studies that examined learning, attention, and speed of processing under the influence of cannabis. Several studies before this one that focused on driving ability looked at how drivers swerve or adapt to a car in front of them.

“This groundbreaking research indicates that cannabis use does impair driving ability, but factors differ from alcohol,” California State Assemblymember Tom Lackey said. “It also underscores the need for further research on this topic. Policymakers still need a better understanding of the effects of different ways of consuming higher concentration products to charter a path forward.”

However, CMCR Co-director and Professor Thomas Marcotte explained that the studies were limited in their ability to represent accurate real-world cannabis usage. 

“Many of those studies were done with occasional users,” Marcotte noted. “So one of the goals of our study was to really approximate real-world circumstances where someone who is a regular user will smoke to a desired highness and we wanted to see how  being acutely high will affect their performance.” 

Furthermore, co-author Professor Robert Fitzgerald noted that the percentage of cannabis in the cigarettes used was higher than the amount of cannabis used in previous studies. 

“Most previous studies used marijuana that was typically 3 or 6% THC by content,” Fitzgerald explained. “We were using a [5.9] and a 13.4% cannabis, which is closer to what’s available in the dispensaries.”

Researchers recruited 191 regular cannabis users, ages 21 to 55 years old, to smoke a cannabis cigarette after abstaining for 48 hours. The percent concentration of cannabis in the cigarette varied in three amounts: 13.4%, 5.9%, and a placebo of 0.02% THC. After smoking, subjects went through a simulation presented on a Driving Simulator System (Systems Technology, Inc) for approximately 25 minutes. The simulation required participants to maintain their lane position and speed in a straight roadway while responding to a divided attention task on an iPad on the dashboard. Throughout the day, after different intervals of time, subjects would go through the simulation again, doing so multiple times. 

Their performance was measured by several different variables, such as the measures of variation of lateral position and speed, and the number of correct divided attention stimuli identified while driving. All of these were calculated into a score to encompass the driving performance; the higher the score, the worse the performance. 

There were several main takeaways from this study. One was that impairment is present even in individuals who are regular users, yet not all of those who were high were significantly impaired: 57 out of the 125 individuals who smoked THC would be classified as impaired at 30 minutes after inhalation. 

Additionally, many people at the beginning reported they were hesitant to drive, Marcotte noted. 

“Roughly 50% of the people would not go on the road after smoking and felt they were too impaired,” Marcotte said.  “However, at 90 minutes after smoking, they started feeling the impairment was wearing off.” 

However, the performance in the simulator did not change from the 30-minute and the one hour and 30-minute marks, despite what subjects claimed. Both groups of smokers with cannabis in the cigarette performed significantly worse than the placebo group at 30 minutes. Additionally, performance didn’t differ between the two different levels of cannabis content, suggesting that the levels of cannabis intake resulted in comparable levels of highness. 

The impairment lasted until about four and a half hours for the entire group, with some individuals feeling less impaired after three and a half hours. However, the paper notes that at three and a half hours, the THC groups’ driving was no longer significantly different from the placebo group’s driving.

Generally, the paper noted there was no relationship between THC blood concentration and impairment. For instance, those who smoked the 5.9% concentration had a higher blood concentration than those who smoked the higher dose, showing that THC content in cigarettes does not yield an expected higher blood concentration. 

Furthermore, the group with the highest cannabis usage in the past six months had significantly higher THC blood concentrations after smoking but didn’t perform worse than those with lower THC blood concentrations; this suggests behavioral tolerance. However, those in this group compensated by ingesting more THC, and thus were still as debilitated as other users. 

“People who are regular users believe that because they use all of the time, they don’t get as stoned with the same amount of THC as someone who’s an infrequent user, and we actually found that is true,” Marcotte explained. “But then they ingested more THC to get that level of highness. So in the end, they’re just as impaired as the other group even though they have developed this tolerance.” 

This finding is significant because this acts as evidence against “per se” laws. Per se laws establish a statutory violation if a legal standard is breached, such as blood-alcohol concentration in driving under the influence laws. This finding shows that when it comes to policing cannabis intake and driving, different laws may have to be made since impairment can’t be deduced by cannabis content in cigarettes, behavioral tolerance, or THC blood concentrations.  

When asked how to police driving under the influence, Fitzgerald stated that he thinks that it comes down to both officer observations of driving performance as well as toxicology testing. 

“Having a set cut point and you’re above this level you’re impaired, you’re below this level you’re not impaired — is not scientifically valid,” Fitzgerald stated. “That comes down to a combination of officer observations along with toxicology testing.”

Marcotte added that the study provides valuable information to users, warning them that they are not always the best judges of their ability to drive. He also noted users should know that a couple of hours after smoking may be the riskiest time to drive. 

“An important thing for users to know is [that] because you’re an experienced user does not necessarily mean you’re going to be less impaired. If you smoke to a level of highness, you still can be a danger,” Marcotte said. 

The paper notes there were several limitations with the study. Despite the sample’s range of how often subjects used cannabis, it didn’t include infrequent users, those who smoke medicinally, and nonusers. Subjects were told to get to a level of highness as they would casually at home, meaning that the study didn’t address controlled dosing or very elevated levels of highness. Additionally, effects on vision and cognition were not measured, and no measurements were taken between an hour and 30 minutes to three hours and 30 minutes after smoking. 

Marcotte stated that there will be an additional study looking into the combined effects of alcohol and cannabis on drivers. 

“Quite often, you do not find cannabis alone in impaired drivers on the road. It’s usually a combination of alcohol or some other substance,” Marcotte explained. “We have a new study starting up to look at the combination of alcohol and cannabis, and seeing whether you can be an alcohol level below the legal limit and officially, you’re fine at least in terms of the blood levels. But what happens when you have some THC onboard as well?” 

In the future, the paper notes that research should look into different factors relating to driving behavior such as biological differences, personal experience with cannabis, and cannabis intake methods. 

In terms of biological differences, Marcotte noted that there may be genetic, metabolic, or sex-based differences in how people process THC, but there have been few studies investigating this. 

Fitzgerald elaborated that they’re interested in studying other methods of ingesting cannabis, such as oral routes like gummies and edibles in addition to highly concentrated forms like vapes and concentrated butane hash oil (also known as dabs). However, he and Marcotte noted that studying vapes and dabs would be difficult on a federal level. 

 “We cannot touch those products because they are federally illegal, but as you know vaping, dabbing gives you a concentrated THC, and there are many edibles that have yet to be investigated,” Marcotte clarified. 

To see more of the research done by the CMCR, you can find their website here. For more information on the effects of cannabis on users, you can refer to this list by UC San Diego Health.