Medical Cannabis: Understanding the Evidence

The Magnitude and Duration of Acute THC-Induced Driving and Cognitive Impairment

Disclaimer: This post presents data from a research paper and is for educational purposes only. The following does not constitute advice as to whether a person should or should not drive and should not be used as a substitute for advice from an appropriately qualified clinician and/or the Driver and Vehicle Licensing Agency (DVLA).

As the availability of legal cannabis products continues to increase, the implications on road safety have consistently been a featuring topic of discussion. Epidemiological and experimental studies have explored the deleterious effects of cannabis use on driving, with findings suggesting that Δ9-tetrahydrocannabinol (Δ9-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, Δ9-THC– exposure can impact driving-related cognitive skills, especially in comparison with data on alcohol consumption. Furthermore, levels of 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, yet unresolved, issues relating to the implications of driving under the influence of cannabis is the duration of Δ9-THC-induced driving impairment. The complex pharmacokinetics of Δ9-THC also mean that, unlike alcohol, there is no clear correlation between Δ9-THC blood concentrations and impairment.

A recent systematic review aimed to characterise the acute effects of Δ9-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 Δ9-THC impairment.

Studies included in the review

Researchers identified 80 publications and 1534 outcomes from the year 2000 until April 2020 using online databases, Web of Science and Scopus. 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 Δ9-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: divided attention, tracing performance, information processing, executive function, reaction time, motor function (sub-categorised as fine and gross motor function), perception (sub-categorised as sensory discrimination and time perception), sustained attention, and working memory. Each driving-related cognitive performance test used in the included studies was categorised into one of these Performance Domains.

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

Meta-analyses of the available data revealed that Δ9-THC had 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 results from meta-analyses confirmed that Δ9-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 conclude 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.

While Δ9-THC exposure was seen to affect time perception – increasing the likelihood of both under- and over-estimation on time estimation and time reproduction tests. The initial meta-analyses also revealed a non-significant trend for a positive effect of Δ9-THC on driving speed – meaning that cannabis consumption was associated with a reduction in average speed. This suggests that drivers may attempt to expand their safety margins – likely due to reduced driving confidence.

This study also identified significant differences between the effects of inhaled Δ9-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 Δ9-THC administration. However, 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 due to their increased tolerability.


The researchers acknowledge several 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 Δ9-THC, route of administration and performance domain. These limitations mean that the current analyses are likely biased toward studies that contributed more effect estimates.

The recovery time after consuming Δ9-THC will depend on several factors, including Δ9-THC dose, method of administration (oral Δ9-THC-induced impairment may take longer to subside).