Smoking CBD-rich cannabis appears to have no significant impact on driving ability despite producing illegal levels of THC in the bloodstream, according to a recent study out of Switzerland.
Previous research into CBD’s impact on driving ability has found that while low doses of CBD has no effect on driving, small doses of THC can cause a brief impairment that wears off in a few hours.
The authors of the Swiss study say that higher CBD concentrations in cannabis seem to reduce the impairment caused by THC.
This suggests that current rules surrounding THC blood levels while driving – which do not take into consideration CBD intake – do not give an accurate picture of impairment.
The study
Cannabis flower with less than 1% THC has been legal in Switzerland for a few years. Which is why researchers in the country were keen to discover if there were any possible risks of consuming CBD-rich cannabis before driving.
For the pilot study, 33 subjects were recruited. Each smoked a joint containing 500 mg of tobacco and either 500 mg of CBD-rich marijuana (16.6% total CBD; 0.9% total THC) or 500 mg of a placebo substance, before performing three different dimensions of the Vienna Test System TRAFFIC.
This test examines reaction time, behaviour under stress, and concentration performance. Three tests of balance and coordination were also conducted, while vital signs (blood pressure and pulse) were measured.
Blood samples were taken after smoking and after completion of the tests to determine the cannabinoid concentrations (CBD, THC and THC-metabolites).
According to the authors, “the results revealed no significant differences between the effects of smoking CBD-rich marijuana and placebo on reaction time, motor time, behaviour under stress, or concentration performance.”
Maximum free CBD and THC concentrations in capillary blood were detected shortly after smoking, ranging between 2.6–440.0 ng/mL and 6.7–102.0 ng/mL, respectively. After 45 min, capillary blood concentrations had already declined and were in the range of 1.9–135.0 ng/mL (free CBD) and 0.9–38.0 ng/mL (free THC).
Although similar THC blood levels have previously been shown to cause signs of impairment when driving, none were observed in the current study.
“This finding suggests that higher CBD concentrations cause a negative allosteric effect in the endocannabinoid system, preventing the formation of such symptoms,“ the study concludes.
Additionally, no effects on vital signs were observed after smoking CBD-rich cannabis.
This study was one of the first to investigate the potential impact of smoking CBD-rich cannabis on driving ability.
Previous research
As mentioned earlier, an Australian study from 2020 found that small doses of CBD appear to have no significant impact on driving, but similar doses of THC were associated with short-term impairment that subsided in about four hours.
There’s also some 2019 research that found no evidence that drivers whose blood samples tested over the legal THC limit (typically between two and five nanograms of THC per millilitre of blood) were more likely to cause a traffic accident.
Taken together, these findings show that THC levels in blood are a very inadequate way of determining impairment and that other methods of detecting impairment should be used.
