Over the past decade, access to legal cannabis products – both medicinal and recreational – has increased significantly across the globe. Cannabis-based medicinal products (CBMPs) can now be prescribed for a wide variety of conditions in many countries around the world, including in the UK; however, some jurisdictions have adopted a more liberal approach to cannabis reform, such as in certain parts of North America. Citizens in Canada and many states in the US are now able to access both medicinal and recreational cannabis products freely. This increased availability has led to growing concern about the impact of cannabis use on driving ability.
Cannabis Use and Driving Impairment
Data shows that there is a rise in the number of individuals who are driving after receiving a prescription for medical cannabis. Like many everyday medicines, driving ability may be reduced (driving impairment) whilst someone is taking medical cannabis. The medication may cause drowsiness and can initially reduce your reaction times. This can increase the risk of having an accident. Individuals prescribed medical cannabis, however, only need to inform the DVLA if they have a notifiable medical condition or disability that could affect their ability to drive safely. They do not need to inform them about their prescription. The only condition that medical cannabis is commonly prescribed which requires an individual to inform the DVLA is epilepsy. However, no matter the medication, individuals should never drive whilst impaired. The dose and prescription can affect individuals differently, so people should always speak to their specialist doctor about possible impairment while taking their medication.
Cannabis contains over 400 cannabinoids, terpenes, and flavonoids; however, the majority of extensive research has placed a decided focus on the properties and effects of cannabinoids. In the case of research assessing driving impairment, the majority of research to date has focused exclusively on THC. Nonetheless, it is recognised that other cannabis compounds may be associated with effects that may lead to driving impairment. For example, the authors of a recent crossover pilot study note that, while CBD is may be associated with sedating effects, these effects may actually be caused by β-myrcene – a terpene that is also commonly found in full-spectrum medical cannabis products.
What is β-myrcene?
β-myrcene is a common terpene that is found at varying concentrations in different cannabis strains. Importantly, though, β-myrcene (like other terpenes) is not exclusive to cannabis. In fact, this particular terpene can also be found in a number of other plants, including basil, lemongrass and hops. β-myrcene is even thought to be the sedating agent in some traditional sleep aids based on hops and lemongrass preparations. Therefore, the authors of the current study aimed to test the hypothesis that β-myrcene can directly impair driving skills.
Design and Methods of the Study
To test this theory, a total of 10 participants (occasional cannabis users) were recruited to take part in a crossover pilot study. The sample was selected because of likely previous exposure to β-myrcene. Each participant attended two sessions: in the first session, participants were given a 15 mg dose of β-myrcene combined with canola oil in a vegetarian capsule. In the second session, participants were given a visually identical capsule containing only canola oil. Neither the participants nor the data collector knew which capsule contained β-myrcene.
At the start of each session, participants completed a 10-minute warm-up drive on the driving simulator to acclimate themselves to the control. They then completed a 20-minute baseline driving session. This was followed by the dosing session, which was then followed by three 20-minute post-dosing simulator blocks. Each 20-minute driving block featured 17 static speed control trials and 16 dynamic speed control trials.
The mean difference between participants’ driving speed and the target speed was recorded for each of the trials. Other factors used to measure driving ability were the standard deviation from lane and the minimum time-to-collision (TTC) which reflects the combination of speed and distance from the vehicle in front and is used as a measure of risk-taking. Divided attention was also measured using timed signals to drivers during periods in which participants were attending to speed limits or the speed of the vehicle in front, creating the need to divide attention.
Results of the Study
The researchers recorded significant differences in driving performance when comparing pre- and post-β-myrcene administration driving sessions. Participants dosed with β-myrcene performed significantly worse than placebo controls on divided attention and static speed control tasks. For example, under β-myrcene, participants made significantly more errors on the divided attention task. However, there was a lack of significant, or even suggestive, findings regarding standard deviation from lane, which is among the measures most sensitive to THC.
The researchers concluded that the results of this study demonstrate proof-of-principle evidence that β-myrcene may contribute to impairment of driving-related skills. However, the authors concede that this pilot study was underpowered, owing to financial limitations and restrictions due to Coronavirus guidelines. In particular, the small number of participants was likely too few to overcome individual differences in responses to β-myrcene. Nonetheless, the results of this study demonstrate that more research is required to fully understand the mechanisms through which particular compounds within the cannabis plant may affect driving ability and safety.
