Combining cannabis edibles with alcohol worsens driving impairment more than either substance alone. In a controlled experiment with 25 healthy adults who reported prior co-use, driving performance was significantly impaired under most active drug conditions, including 25 mg THC with alcohol at 0.05% breath alcohol concentration. The legal alcohol intoxication limit of 0.08% may be too high when cannabis has also been consumed. Standard field sobriety tests often failed to detect impairment that was evident in driving measures. The findings suggest a need for better impairment detection and policies that account for co-use.
Caffeine co-administered with THC produced minimal changes in subjective effects, performance, or metabolism, though signals for perceived driving impairment were observed. In contrast, CBD co-administered with THC and caffeine increased outcomes associated with abuse liability and performance impairment versus THC alone. CBD also increased plasma THC and 11-OH-THC concentrations. These data provide the first direct assessment of pharmacodynamic and pharmacokinetic effects of THC and caffeine co-administered in humans, emphasizing the importance of considering full cannabinoid profiles and drug combinations in regulatory decision-making.