Cannabis use can “significantly” impair drivers for up to four hours after use, a new study revealed, stressing the urgency for a better understanding of the effects its use increases around the country. The randomized clinical trial of 191 regular cannabis users in the study by University of California, San Diego, the FDA and the Research Advisory Panel of California published last week in the JAMA Psychiatry journal determined that THC consumption “comprised key driving simulator variables, assessed prior to smoking and at multiple time points post-smoking.”