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Study Shows Cannabis Impairment Subsides in Four Hours

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Recreational cannabis consumers have been recommended to wait at least seven hours after smoking before engaging in driving or other sensitive tasks where safety is of the utmost importance.

However, a new review of clinical studies looking only at medical cannabis consumers have concluded that the duration of impairment lasts less than four hours. This finding has significant implications for people who drive or work in potentially dangerous occupations. 

The study determined the duration of acute neurocognitive effects of consuming medical cannabis and the difference between medical and recreational users. It reviewed clinical studies and other systematic reviews of patients prescribed medical cannabis for chronic non-cancer pain or muscle spasticity. Medical cannabis has been shown effective for these two particular conditions. 

The authors of the study concluded that impairment of cognitive and motor tasks depended on the dose of delta-9 tetrahydrocannabinol (THC) and decreased following consumption to the point where there was no difference in cognitive performance after four hours between those taking THC and those taking a placebo. THC is known to have acute adverse effects on attention, coordination and reaction time. 

THC is also prescribed to manage symptoms for some conditions like PTSD and neurological disorders. The authors of the study say that these impairments may improve due to the use of medical cannabis in some cases.

Until now, guidelines about how cannabis affects cognition and psychomotor abilities have come from studies solely focused on recreational consumption. Medical cannabis patients need advice tailored to their conditions and patterns of use—which is often very different from those of recreational consumers. 

It’s also common for patients to consume edible forms of cannabis, meaning the onset and duration of effects take longer than after inhalation. They usually start with lower doses of THC and increase only as needed to avoid unwanted side effects. Medical cannabis products often contain cannabidiol, which is (in most cases) non-intoxicating and may mitigate at least some THC effects.