Rationale:  Cannabis use has been linked to driving impairments, a special concern for older drivers who are already susceptible to hearing loss, poor eyesight, and slower cognition. The purpose of this study was to determine how THC in the blood is related to driving abilities in older adults. 

Method: Thirty-one regular cannabis users, aged 65-79 years old, came to the laboratory two times, at least 3 days apart. In one session, participants were instructed to bring their own cannabis and to smoke their usual amount. As a control, they did not use cannabis during the other session.

On both days they had their driving assessed after 30 and 180 minutes in a car simulator. To simulate distracted driving, participants were also tested while they counted down by 3’s. Before, during, and after the driving tests, blood levels of THC were assessed.

Results: 30 minutes after smoking, regular users weaved more, both with and without distractions, compared to when they were “sober.” Regular cannabis users also drove more slowly when intoxicated.

Meaning: Even regular, frequent users, who are likely to be tolerant to some effects of THC, showed impaired driving after using. These effects may lead to more car accidents.