How long does a cannabis high really last? There is, it appears, no definitive answer. Which makes it really difficult for the pharmacist to tell you how much to take, when used for medicinal purposes. And still scientists have managed to at least narrow down the timeframe to between three and 10 hours. Knowing even this much about the timeframe of a high can help drivers to time doses in order to avoid driving while under the influence.

The new understanding of how long cannabis affects us come from metadata data pulled from 80 studies. Of course, how long you stay high depends on factors such as the form of cannabis you consume, and the strength of the cannabis in question. All in all however, a user can use this new information to avoid doing tasks that require more focus and reaction time.

Does the new timeframe mean that you’re sure to pass a drug test after ten hours? The short answer is drivers beware—the answer is no. According to University of Sydney Psychopharmacologist Iain McGregor, “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,” says study author McGregor, whose meta-analysis was published in Neuroscience & Biobehavioral Reviews.

Up in Smoke or Down the Hatch

The high does in part depend on how you take your medical cannabis. Do you smoke it or take it by mouth? Do you use it on a regular basis? Do you use a mild dose? All these factors can make a difference to a driver’s safety and that of their passengers and others on the road.

This was a fairly large sample the scientists looked at, some 1,534 “performance outcomes,” meaning people who got high and then drove or did some other comparable cognitive task, at some point after taking cannabis.

“Our analysis indicates that impairment may last up to 10 hours if high doses of THC are consumed orally. 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,” says McCarthy.

“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.”

Regular Cannabis Use

Then again, the researchers note that regular users of cannabis tend to develop a tolerance to the medication, which can improve their performance at such complicated tasks as driving. On the other hand, as a regular user builds up a tolerance, they are more likely to increase the dose to get the same level of high as before.

“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,” says U of Sydney Behavioral Pharmacologist Thomas Arkell.

The team of researchers for this meta-analysis concluded that most of the skills needed to drive a car are back to normal within, more or less, five hours after smoking up.

Legislative Implications

And what are the implications of all this for legislation?

“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,” says McGregor.