A comprehensive analysis of 80 scientific studies has identified a “window of impairment” between three and ten hours caused by moderate to high doses of the intoxicating component of cannabis, tetrahydrocannabinol (THC). The results have implications for the use of drug laws around the world, researchers say.

The study found that the exact duration of the impairment depends on the THC dose, whether the THC is inhaled or ingested orally, whether the cannabis user is regular or occasional, and what demands are placed on the task while drunk.

The study is the first such meta-analysis and summarizes the results of 80 separate scientific studies on THC-induced impairment that have been carried out over the past 20 years. It was published in Neuroscience & Biobehavioral Reviews.

“Legal cannabis use, both medicinal and non-medicinal, is increasing worldwide,” said lead author Dr. Danielle McCartney of the Lambert Cannabinoid Therapeutics Initiative at the University of Sydney.

“THC is known to acutely affect driving and cognitive performance, but many users are unsure how long this impairment will last and when they can resume safety-related tasks such as driving after using cannabis.

“Our analysis shows that impairment can last up to 10 hours when high doses of THC are taken orally. However, a more typical duration of impairment is four hours when lower doses of THC are ingested by smoking or vaping and performing simpler tasks (e.g. those who use cognitive skills such as reaction time, sustained attention, and working memory).

“This impairment can last up to six or seven hours when higher doses of THC are inhaled and complex tasks like driving are assessed.”

For this study, a moderate dose of THC is around 10 milligrams, but researchers say that what is moderate for a normal user might be high for an occasional user.

The co-author Dr. Thomas Arkell, also of the Lambert Initiative, said: “We have found that impairment is much more predictable in casual cannabis users than normal cannabis users. Heavy users show a significant tolerance for the effects of cannabis on driving and cognitive function usually some impairment. “

The authors found that regular users may consume more cannabis in order to have an effect, which leads to a corresponding impairment.

Many medicinal cannabis users consume THC in the form of oils, sprays, or capsules. Another important finding was that the impairment with such oral application is longer and significantly longer than with inhalation.

The researchers said the results have implications for what are known as drug driving laws.

Lambert Initiative Academic Director Professor Iain McGregor said, “THC can be detected in the body weeks after cannabis use, while it is clear that the impairment is much shorter. Our legal framework will likely need to make up for it. Focus as it is with alcohol on the interval at which users pose a greater risk to themselves and others, and prosecution based solely on the presence of THC in blood or saliva is clearly unfair.

“Laws should be about street safety, not arbitrary punishment. With cannabis legal in more and more jurisdictions, we need an evidence-based approach to drug driving laws,” said Professor McGregor.

This paper follows recent research by Dr. Arkell and colleagues showing that cannabidiol (CBD), one of the medically active components of cannabis, does not cause impairment in driving behavior.


Journal reference:

McCartney, D., et al. (2021) Determination of the extent and duration of driving and cognitive impairment induced by acute Δ9-tetrahydrocannabinol (Δ9-THC): A systematic and meta-analytical review. Neuroscience & Biobehavioral Reviews. doi.org/10.1016/j.neubiorev.2021.01.003.