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How Long Does a Cannabis High Last?

Researchers in Australia release findings on cannabis impairment.

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One of the longstanding questions for anyone who has ever used cannabis is this: How long am I going to feel like this? It’s also important information to have when people try to decide how long they should wait between using cannabis and getting behind the wheel.

For lawmakers, more precise information on cannabis impairment when it comes to driving could help them craft laws that are fair - or, at the very least, laws that don’t punish people for smoking weed days before getting pulled over.

That’s the hope of researchers in Australia, who recently released the findings of a study that analyzed data on cannabis impairment from the past two decades. While many factors can influence the level and length of impairment, the study found that the most hours someone might be impaired after using cannabis is 10 hours, the shortest time is three hours, and four hours is the most typical timeframe.

The researchers believe their findings can help lawmakers establish more accurate rules around when a person should not get behind the wheel after using cannabis. It’s an issue that has plagued law enforcement for years.

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How high is too high to drive?

Law enforcement is infamously unable to determine whether a person is too impaired to drive after using cannabis. That’s because THC, the ingredient in cannabis that gets you high, stays in the bloodstream long after the effects have worn off.

Science has settled the question. A 2020 study that CBD does not impair drivers also reported that, while it can stay in the bloodstream for many days, THC does not impair people for longer than four hours and the effects are mild after 40 minutes.

But in Australia, people with THC in their bloodstream can get fined or even lose their license, even if the effects wore off days ago. 

Researchers involved in the new study from Australia, done by the Lambert Initiative for Cannabinoid Therapeutics at the University of Sydney, set to help correct that issue. Iain McGregor, the academic director of the Lambert Initiative, told ABC News Australia that medical cannabis users in the country obviously have a lot of interest in the issue.

“You’ve got this massive amount of a prescription drug going into people who are told, 'You can't drive at all, you can't even have one molecule of THC in your system', which is, you know, just ridiculous," he said.

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Four to five hours is the most frequent choice for impairment time.

The Australian research involved analysis of data in 80 different scientific studies conducted in the last 20 years that measured the effect of THC on driving. Like the earlier research, it reached the conclusion that about four hours is the typical time of impairment.

However, researchers used a three-hour to 10-hour window because many factors can impact how long you’re impaired. Those factors include:

  • The size of the dose
  • The method it is taken (orally or inhaled)
  • How often you use cannabis (your tolerance level)

Danielle McCartney, the study's lead researcher, told ABC that the most typical time of impairment is between four and five hours. The 10-hour impairment happened when people used cannabis through edibles, she said.

While the research can help people know how long they will be impaired and should not get behind the wheel, McCartney said she also hopes the research will help officials “make policies that are evidence-based and tell people how long they should wait before driving.”

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