A study published in the Journal of Cannabis Research finds that frequent marijuana users did not show measurable driving impairment the morning after use, even when detectable levels of THC remained in their blood and oral fluid.

The research was conducted by scientists from the Institute for Mental Health Policy Research, Trent University and the University of Toronto. The study examined whether marijuana consumed the night before could still affect driving performance the following morning.

Researchers recruited 65 frequent marijuana users who reported smoking at least four times per week, along with a matched control group of 65 people who do not use marijuana. Participants were similar in age, sex, driving experience, yearly driving distance and sleep patterns.

Frequent users completed a driving simulator test 12 to 15 hours after their last marijuana use. During the same session, researchers collected blood and oral fluid samples to measure THC, cannabidiol (CBD) and related metabolites.

The analysis found no statistically significant differences in driving performance between the marijuana users and the control group. Measures such as vehicle speed, following distance and lane control were similar between both groups during both standard driving conditions and more demanding distracted driving scenarios.

The standard deviation of lateral position, a measure often used to assess lane weaving, was slightly higher in the non-marijuana group than in those who had used marijuana the night before, though the difference was small and not statistically significant.

Researchers also found no meaningful relationship between THC concentrations in blood or oral fluid and driving behavior. Average blood THC levels among participants were above 2 nanograms per milliliter at the time of testing.

Although some marijuana users reported feeling more intoxicated and expressed greater willingness to drive before the simulator session, those differences also were not statistically significant after statistical corrections.

The researchers conclude that frequent marijuana users did not show significant driving impairment 12 to 15 hours after use. They say larger studies are needed to determine whether the same findings apply to occasional users or to other forms of marijuana consumption.