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The lingering effects of cannabis can last for hours. Tegra Stone Nuess/Getty Images
  • Researchers say cannabis can impair a person’s driving ability for as much as 4 hours after the drug is used.
  • Experts say the lingering effects of cannabis vary from person to person and can even be influenced by what you eat while using the drug.
  • They add that people should know how cannabis affects their body before they decide to get behind the wheel after smoking or ingesting the drug.

A study published today says cannabis affects a person’s ability to drive for as much as 4 hours after ingestion.

Researchers from the Center for Medicinal Cannabis Research at the University of California San Diego looked at 191 regular cannabis users, 118 of whom were men. They found that smoking cannabis “resulted in simulated driving decrements.”

“Worse driving performance is evident for several hours post-smoking in many users but appears to resolve by 4 hours 30 minutes in most individuals,” the study authors wrote. “Further research is needed on the impact of individual biologic differences, cannabis use history, and administration methods on driving performance.”

Researchers also said that when experienced cannabis users control their own intake, “driving impairment cannot be inferred based on THC content of the cigarette, behavioral tolerance, or THC blood concentrations.”

The study authors also said participants’ increasing willingness to drive an hour and 30 minutes after ingestion “may indicate a false sense of driving safety.”

The researchers indicated that the importance of knowing more about the effect of cannabis on driving is becoming more important as more states legalize cannabis for medical and recreational use.

The study participants were 21 to 55 years old, used cannabis four or more times a month, had a valid drivers’ license, and drove at least 1,000 miles the year before the study began.

“The psychoactive component of cannabis that can impair driving is THC,” said Dr. Monty Ghosh, an addiction specialist and assistant clinical professor at the University of Alberta Hospital in Canada.

“When smoking, the onset of the effects (is) felt at 15 to 30 minutes and can last up to 4 hours,” Ghosh told Healthline. “This differs when someone ingests cannabis, such as with edible formulations which can last longer in one’s system, up to 12 hours. Doses of cannabis can impair attention, concentration, short-term memory, and are directly proportional to the amount and concentration smoked.”

Ghosh said the effects of cannabis on driving can be more dramatic than alcohol, which “very much” depends on not only how much a person drinks but their ability to metabolize alcohol. He added alcohol may be easier to detect.

“With alcohol, we have breathalyzer testing, which can help with determining if someone has unsafe blood alcohol levels which can impair driving,” he said. “With cannabis, we do not have sufficient testing methods to see how much someone has smoked and if they have a concerning concentration of cannabis in their system.”

Liz Rogan is a certified cannabinoid medicine specialist and founder of the Cannabis Business Council of Santa Barbara County in California.

She told Healthline how cannabis affects driving relies on how it’s ingested, the compounds within the cannabis, and the user’s body.

She said the effects could even depend on what a person consumes with the cannabis.

“For example, chocolate has compounds that can extend or magnify the effects of cannabis,” she explained.

Rogan said knowing that someone’s too impaired to drive isn’t as cut and dried as police using technology to assess how much alcohol a driver has consumed.

“(Police) gauge it by smell, sight, or if the person is disobeying traffic laws, but they look for signs like bloodshot eyes, nervous demeanor, erratic or very slow driving, especially if there is an accident involved,” Rogan said. “They still use field sobriety tests for suspected cannabis impairment because neither breathalyzers nor blood tests can accurately assess this.”

“There isn’t an exact test at this time, but it’s in the works,” Rogan added. “One that is potentially indicative of recent cannabis use tests saliva. The challenge is that the level of THC in your blood is not necessarily indicative of effects, nor impairment.”

Rogan said users should try to assess how cannabis affects their body before using, if possible.

“The best practice is to figure out your tolerance before you decide to drive and remember that alcohol and other substances combined with cannabis may also have an additive impact on impairment,” she said. “Always take stock of how you feel before you get behind the wheel. This will help get you to your ‘safe place’ when you decide to drive after cannabis consumption.”