It’s not easy to tell how impaired a person will be based on how much THC they have in their body a study has found. The research was funded by a National Institute of Justice grant awarded to RTI International scientists who carried out the research.
In the research, 20 people who either ate or vaporized cannabis with varying levels of THC were then tested with sobriety and cognitive tests.
Study participants’ cognitive and psychomotor functioning were negatively impacted after all oral and vaped doses of cannabis over 5 milligrams.
For vaped THC doses over 5 milligrams, the greatest cognitive and psychomotor effects were observed between zero and two hours, returning to baseline after four hours.
For orally consumed THC doses, cognitive and psychomotor effects were observed one hour after administration and peak effects were seen about five hours after administration. It took longer with oral THC for functioning to return to normal — 8 hours.
However, even though all those that received doses higher than 5 milligrams of THC experienced observable impairment, THC levels in body fluids didn’t correlate accurately with indicators of intoxication and impairment.
In other words, some participants with higher levels of THC had low levels of impairment and the opposite also occurred.
“Many of their study participants had significantly decreased cognitive and psychomotor functioning even when their blood, urine, and oral fluid contained low levels of THC,” the RTI International researchers said.
In addition, the study found that standardized field sobriety tests commonly used to detect driving under the influence of drugs or alcohol, were useless at detecting marijuana intoxication. In a tweet the NIJ said that various tests, including standing on one leg, balancing and walking and turning, “were not sensitive to cannabis intoxication for any of the study participants.”
A previous study found that low-level THC consumption has an impact that is "modest in magnitude and like that seen in drivers with a 0.05%” blood alcohol concentration”.
Of course, this doesn’t mean that it is safe to drive after taking cannabis — if anything this study suggests using extra caution as there’s no telling how strongly you will be impaired.