24th September 2021
By Emily Ledger
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As recreational cannabis products containing significant levels of THC become more easily available, the implications of legal recreational markets have consistently been a featuring topic of discussion.

As a result, a number of epidemiological and experimental studies have explored the deleterious effects of cannabis use on driving. The results of these studies suggest that THC exposure can lead to a moderately increased risk of drivers being involved in, or responsible for, a car crash.

However, there is still a relatively low level of understanding around how cannabis – and particularly, THC – exposure can impact driving-related cognitive skills – especially in comparison with data on alcohol consumption.

The inconsistencies of THC impairment

Levels of THC-induced impairment have been seen to vary significantly, with the results of experimental studies suggesting that impairment is less pronounced in regular cannabis users – likely due to the development of tolerance.

One of the key issues relating to the implications of driving under the influence of cannabis is the duration of THC-induced driving impairment. The complex pharmacokinetics of THC means that it varies from alcohol, for which blood concentrations directly correlate with impairment.

A recent systematic review aimed to characterise the acute effects of THC – the most common psychoactive cannabinoid produced by the cannabis plant – on driving performance and driving-related cognitive skills and to determine the duration of THC impairment.

Existing evidence assessing THC impairment

Using online databases, researchers identified 80 publications and 1534 outcomes that were carried out between 2000 and 2020. Studies that “measured either simulated or on-road car driving performance, or a discrete cognitive skill related to car driving, ≤12 h following a single, acute dose of THC in a placebo-controlled (within- or between-subject) experimental trial were eligible for inclusion.

Driving-related cognitive skills – referred to in the review as Performance Domains included (1) Divided Attention; (2) Tracing Performance; (3) Information Processing; (4) Executive Function; (5) Reaction Time; (6) Motor Function (sub-categorised as Fine and Gross Motor Function); (7) Perception (sub-categorised as Sensory Discrimination and Time Perception); (8) Sustained Attention; and (9) Working Memory.

Each driving-related cognitive performance test used in the included studies was categorised into one of these Performance Domains.

A total of 155 trials, with a total of 3454 participants, derived from 80 original research studies were included in the systematic review.

Results of included studies

Cognitive Performance

A meta-analysis of the existing data reveals that THC exposure likely has significant detrimental effects on Divided Attention, Tracking Performance, Information Processing, Conflict Control, Fluid Intelligence, Reaction Time, Fine Motor Function, Sustained Attention and Working Memory. However, neither Sensory Discrimination nor Time Perception demonstrated significant impairment.

Car Driving Performance

The findings from this review also confirmed that THC impairs aspects of driving performance and demonstrate that “the magnitude and duration of this impairment depend on the dose provided, route of administration and frequency with which cannabis is used.”

Therefore, the researchers concluded that there appears to be no universal answer to the question of “how long to wait before driving?” following cannabis use.

Several measures of driving performance also exhibited significant impairment, while significant changes were not identified in other skills, such as Car Following Headway and Variability, Speed, Speed Variability, Sensory Discrimination, and Time Perception. However, the researchers warn that these results should be interpreted with caution as relatively few studies were available in these domains.

THC exposure was seen to affect Time Perception – increasing the likelihood of both under- and over- estimation of time in time estimation and time reproduction tests.

However, an initial meta-analysis of the existing literature revealed a non-significant trend for a positive effect on driving speed – meaning that cannabis consumption was associated with a reduction of speed and suggesting that drivers impaired by THC may seek to expand their safety margins, likely due to decreased driving confidence.

This study also identified significant differences between the effects of inhaled THC in ‘Regular’ and ‘Other’ (mostly occasional) cannabis users. Regular cannabis users (i.e. weekly or more often) was associated with less cognitive impairment following acute THC administration.

Nonetheless, it is important to note that while ‘regular’ cannabis users demonstrated less impairment to driving ability, it is possible that they usually use higher doses than those used in the study, due to their increased tolerability.

Conclusions

The researchers acknowledge a number of limitations relating to the studies included in the review. These limitations include the exclusion of non-English language articles; and the general assumption of a fixed rate of recovery at a given dose of THC, route of administration and performance domain. These limitations mean that the current analyses are likely biased toward studies that contributed more effect estimates.

Nonetheless, the current findings suggest that individuals should wait at least 5 hours following the consumption of inhaled cannabis before performing safety-sensitive tasks, such as driving and operating heavy machinery. However, they also note that recovery time will depend on several factors, including THC dose, method of administration (oral THC-induced impairment may take longer to subside).

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