A comprehensive analysis of 80 scientific studies has identified a ‘window of impairment’ of between three and 10 hours caused by moderate to high doses of tetrahydrocannabinol (THC), the cannabis component that causes intoxication. According to the researchers, these results have consequences for drug-driving laws around the world.
How long the impairment lasts depends on the THC dose, whether it is taken orally or inhaled, on the usage habits of the cannabis user and the demands of the task. The psychoactive THC component of cannabis has potential medical applications in treating nausea, sleep apnoea, fibromyalgia and chronic pain, though these applications are controversial and currently difficult to study due to legal issues, though off-label use is common.
Previous research by Dr Arkell and colleagues has shown that cannabidiol (CBD), one of the medically active components of cannabis, does not cause impairment in driving. CBD has analgesic and anti-inflammatory actions, as well as anxiolytic, antiemetic, antipsychotic, and neuroprotective antioxidant properties
Medical and non-medical legal cannabis use is on the rise worldwide.
THC causes acute impairment in driving and cognitive performance, but there is uncertainty among users about the duration of this impairment and when they can start tasks such as driving after consuming cannabis.
“Our analysis indicates that impairment may last up to 10 hours if high doses of THC are consumed orally,” said lead author Dr Danielle McCartney, Lambert Initiative for Cannabinoid Therapeutics at the University of Sydney. “A more typical duration of impairment, however, is four hours, when lower doses of THC are consumed via smoking or vaporization and simpler tasks are undertaken (eg, those using cognitive skills such as reaction time, sustained attention and working memory). This impairment may extend up to six or seven hours if higher doses of THC are inhaled and complex tasks, such as driving, are assessed.”
A moderate THC dose is considered about 10 milligrams in this study, but could be higher for a regular user, said the researchers.
Co-author Dr Thomas Arkell, also from the Lambert Initiative, said: “We found that impairment is much more predictable in occasional cannabis users than regular cannabis users. Heavy users show significant tolerance to the effects of cannabis on driving and cognitive function, while typically displaying some impairment.”
Regular cannabis users might consume more to get the same effect, resulting in equivalent impairment, the authors noted.
In the case of oral use as in medical cannabis drops, tablets etc, the impairment takes longer to manifest and has a longer duration than the inhalation route.
The findings have implications for so-called drug-driving laws, the researchers said.
Professor Iain McGregor, Academic Director of the Lambert Initiative, said: “THC can be detected in the body weeks after cannabis consumption while it is clear that impairment lasts for a much shorter period of time. Our legal frameworks probably need to catch up with that and, as with alcohol, focus on the interval when users are more of a risk to themselves and others. Prosecution solely on the basis of the presence of THC in blood or saliva is manifestly unjust.
“Laws should be about safety on the roads, not arbitrary punishment. Given that cannabis is legal in an increasing number of jurisdictions, we need an evidence-based approach to drug-driving laws,” Prof McGregor said.
Source: News-Medical.Net
Journal information: McCartney, D., et al. (2021) Determining the magnitude and duration of acute Δ9-tetrahydrocannabinol (Δ9-THC)-induced driving and cognitive impairment: A systematic and meta-analytic review. Neuroscience & Biobehavioral Reviews. doi.org/10.1016/j.neubiorev.2021.01.003.
