Cannabis users should wait at least five hours after smoking a joint before attempting to drive a car, according to a new study into the duration of impairment caused by the drug. However, the study authors stress that this period may need adjusting under certain conditions, and that people who ingest cannabis orally may remain stoned for as long as eight hours.
Presenting their findings in the journal Neuroscience and Behavioral Reviews, the researchers sought to provide evidence that could assist the authorities in devising laws regarding driving after cannabis use. To do so, they conducted a review of 80 previous studies into the drug’s impact on driving ability, involving a total of 1,534 participant outcomes.
Analyzing the recorded data, they found that tetrahydrocannabinol (THC) – the main psychoactive component of cannabis – impairs a number of aspects of driving, including reaction time and the ability to remain in the center of a lane. As blood levels of THC peak, several driving-related cognitive skills are also compromised, with motor function, working memory, and fine motor skills being among the most affected.
However, the majority of these skills tend to recover around three hours after smoking a moderate amount of THC, which the study authors define as 10 milligrams, with driving ability virtually back to regular levels after five hours. Those who smoke twice this dose, meanwhile, generally see most of their capacities restored after five hours, and a near-complete recovery after seven hours.
Yet the study authors concede that “there appears to be no universal answer to the question of 'how long to wait before driving?'", as a number of factors appear to influence the amount of time it can take for the effects of cannabis to wear off. Mode of ingestion, for instance, has a significant impact on the drug’s effects, and those who orally consume cannabis oils or edibles should expect to remain stoned for considerably longer.
Furthermore, people who use cannabis at least once a week tend to experience less impairment than those who do so less often, possibly because of the fact that regular use results in a downregulation of the cannabinoid 1 receptor, which is the main binding site for THC in the brain.
“Our analysis indicates that impairment may last up to 10 hours if high doses of THC are consumed orally,” explained study author Dr Danielle McCartney in a statement. “A more typical duration of impairment, however, is four hours, when lower doses of THC are consumed via smoking or vaporisation and simpler tasks are undertaken (e.g., 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.”