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/operate heavy machinery and should not be used as a substitute for advice from an appropriately qualified clinician, an occupational health assessment, and/or the Driver and Vehicle Licensing Agency (DVLA).
Cannabis is the most commonly used drug in the world, with an estimated two hundred million people consuming it for intended recreational and/or medicinal purposes each year. While the medicinal use of cannabis has a history dating back thousands of years, decades of prohibition featured significant debate around the potential benefits and harms of the plant and its derivatives. Perhaps the most talked-about concern regarding cannabis use is the effect of delta 9-tetrahydrocannabinol (THC), one of the most common constituents of the plant.
THC is one of a large number of compounds known as cannabinoids. This particular cannabinoid may induce intoxication when consumed at sufficient concentrations. Intoxication due to THC consumption has been found to impair both cognitive function and psychomotor performance, both of which may increase the risk of error, accident, or injury when engaging in safety-sensitive tasks such as driving or operating machinery. Therefore, whilst patients who are prescribed medical cannabis can continue to drive if they do not have any other reason that would preclude them from doing so, they are counselled to never drive and/or operate machinery if they feel impaired.
However, there remains some debate about how long THC-related impairment continues to affect people after consumption, and therefore, how long an individual should wait before performing safety-sensitive tasks if they do feel impaired after administering their medication. A recent meta-analysis sought to determine if there was a “window of impairment” after THC consumption. Unfortunately, the majority of the studies included in this review did not include medical cannabis patients and therefore the researchers suggest that their findings may not be directly applicable to medical cannabis patients. However, the researchers noted that the exact time frame of impairment was dependent on several factors:
- dose: higher THC doses produced longer-lasting impairment;
- route of administration: oral THC produced longer-lasting impairment than inhaled THC, and;
- regularity of cannabis use: occasional cannabis consumers became more impaired than regular cannabis users.
Despite these findings, some government agencies in the US warn that THC impairment may persist for over 24 hours and recommend that individuals wait for at least this long before performing safety-sensitive tasks, including driving. Such guidelines could significantly impact the lives of medical cannabis patients, particularly those who are reliant on driving and/or operating machinery for their work. It is, therefore, important for lawmakers and employers to implement regulations that have been informed by a comprehensive review of the available scientific evidence.
A recent systematic review aimed to analyse the current evidence gathered by studies investigating the “next day” effects of THC.
Design and Methods of the Study
For this systematic review, researchers identified relevant studies by searching online databases Scopus and Web of Science dated until March 28, 2022. Studies that met the eligibility criteria were further screened by investigators and the reference list of each study was searched to ensure all relevant articles were included.
A total of 21 studies that measured participant performance on safety-sensitive tasks and/or neuropsychological tests >8 hours after THC (or cannabis) use were included in the final review. The data assessed in this review were used to determine whether the performance tests administered over 8 hours after cannabis use were affected by the acute effects of THC. The results were described in terms of whether THC was found to have a significant impact on each performance test when compared to either placebo or baseline results (dependent on the design used in each study).
Neuropsychological tests and safety-sensitive tasks performed across all studies aimed to assess divided attention, executive function, information processing, tracking performance, reaction time, motor function, sustained attention, working memory, perception, learning and/or memory, spatial reasoning, simulated driving, and simulated flying.
Results of the Study
A total of 458 participants across 20 studies were included in the systematic review, with no studies including participants from clinical populations. No studies were thought to be at ‘low risk’ of bias, with 45% of studies having ‘some concerns’, whilst 55% studies had a ‘high risk’ of bias.
A total of 180 neuropsychological tests and 29 safety-sensitive tasks across 16 of the included studies showed no “next day” effects of THC. These tests were conducted between > 8-12, >12-24, and >24-48 hours after cannabis administration. Most of these studies (45%) used randomised, double-blind, placebo-controlled designs and administered cannabis by smoking (55%).
In comparison, a total of 10 neuropsychological tests conducted between > 8-12 hours post-cannabis administration and two safety-sensitive tasks conducted 24 hours post-administration indicated negative “next day” effects of THC. These negative effects were observed across a total of five studies. None of these studies used randomised, double-blind, placebo-controlled designs.
This systematic review identified little high-quality scientific evidence to suggest that cannabis consumption impairs “next day” performance of safety-sensitive tasks. Of the 345 performance tests reviewed, only 3.5% indicated negative or impairing effects associated with previous day use of cannabis.
However, the researchers note that, while two recent studies, both finding no “next day” effects of cannabis, were identified as having good-quality research methods, 50% of studies with similar findings were categorised as having a high risk of bias. Furthermore, 42% of tests that signified no ”next day” effects also failed to demonstrate “acute” (< 8 hours post-treatment) THC-induced impairment. This is important as ‘‘next day’’ effects seem unlikely to occur in the absence of initial impairment.
The Researchers’ Conclusions
The authors of this systematic review note that, while the data assessed in this study should be interpreted with caution, the existing evidence appears to offer little support for the assertion that cannabis use has a significant “next day” effect on the performance of safety-sensitive tasks. In fact, higher-quality evidence suggests that cannabis use does not significantly affect cognitive function and safety-sensitive tasks. Nonetheless, the authors recommend that further research, “in particular, studies involving both occasional and medicinal cannabis users and oral THC administration”, be conducted to improve our understanding of this issue. Most importantly, medical cannabis patients should still continue to avoid driving or operating heavy machinery if they feel impaired, even if more than 8 hours have elapsed since their most dose of medication.
