Regular cannabis users showed no significant impairment when driving a simulator 12 to 15 hours after smoking cannabis, according to new research that raises questions about current legal limits for THC in blood.
The early access study, published in the Journal of Cannabis Research, compared 65 frequent cannabis users who had smoked the night before with 65 non-users. Despite mean blood tetrahydrocannabinol (THC) concentrations remaining above 2 ng/mL – the legal limit in the UK – researchers found no significant differences in driving performance between the two groups.
“The regular cannabis use group showed no significant impairment in driving performance 12-15 hours after last cannabis use the night before, compared to the control group,” the study concluded. “Blood and oral fluid THC concentrations may not be an accurate correlate of driving behaviour.”
The findings suggest that current legal thresholds for THC may not accurately reflect actual driving impairment, potentially criminalising users who pose no road safety risk.
Participants in the cannabis group smoked products containing an average of 30% THC and 0.6% cannabidiol (CBD), inhaling approximately 159 mg of THC. They completed four driving scenarios measuring weaving, speed control, reaction time and following distance.
The study measured standard deviation of lateral position (SDLP) – the amount of “weaving” a driver exhibits – which is considered the most consistent indicator of cannabis impairment. Under both standard and distracted driving conditions, the control group actually showed slightly higher SDLP than the cannabis group, though these differences were small and not statistically significant after accounting for multiple comparisons.
No other driving measures, including speed, following distance or reaction time, showed significant differences between groups. The number of collisions was too low to analyse.
The research comes as UK medical cannabis patients face an uncertain legal landscape around driving. UK law maintains a strict 2 µg/L (2 ng/mL) blood THC limit, with police able to test oral fluid at roadside and take blood samples if impairment is suspected.
A statutory defence exists for patients prescribed medicinal cannabis provided they hold a valid prescription, follow dosage instructions and show no driving impairment, however patients have reported ongoing threats of prosecution and a lack of understanding amongst police officers. New national guidance issued in 2026 for England and Wales’ 43 police forces instructs officers to assume lawful medical use for prescribed cannabis patients, who are viewed as vulnerable people with chronic conditions.
Interestingly, cannabis users participating in the study reported significantly higher ratings on measures of intoxication, answering “I feel high” at levels around 30 out of 100 on visual analogue scales. They also believed cannabis had affected their driving ability more than the control group did – yet their actual performance showed no impairment.
This disconnect between subjective feelings of intoxication and objective performance raises important questions about self-assessment of fitness to drive. Before driving, cannabis users reported higher willingness to drive in certain conditions, though these differences disappeared after the driving session.
The study also found no correlation between the percentage of THC in the smoked cannabis and any measure of driving performance. Similarly, neither blood THC nor oral fluid THC concentrations correlated with driving impairment, though researchers noted a trend towards correlation with 11-hydroxy-THC (11-OH-THC), an active metabolite.
The researchers emphasised that their findings apply specifically to frequent users – those who smoke cannabis four to seven times per week. Most participants in the cannabis group were daily users who had been consuming cannabis for an average of 12 years, potentially developing tolerance to its effects.
“Future large-scale studies are also needed to determine whether people who use cannabis less frequently are impaired by cannabis when they smoke cannabis the night before,” the study noted. Occasional users may experience more pronounced or longer-lasting effects.
The study also highlighted that different consumption methods may have varying effects. Edible cannabis products produce longer-lasting blood THC concentrations than smoked cannabis, suggesting their effects on driving could persist longer, though this remains largely unstudied.
The research adds to a growing body of evidence questioning the relationship between THC concentrations and actual impairment. While it’s well-established that cannabis affects driving in the hours immediately after use, the exact duration of these effects and their correlation with THC levels remains contested.