Effects of fexofenadine, diphenhydramine, and alcohol on driving performance - A randomized, placebo-controlled trial in the Iowa Driving Simulator

Background: Sedating antihistamines may impair driving performance as serio usly as alcohol. Objective: To compare the effects of fexofenadine, diphenhydramine, alcohol , and placebo on driving performance. Design: Randomized, double-blind, double-dummy, four-treatment, four-period crossover trial. Setting: The lowa Driving Simulator. Participants: 40 licensed drivers with seasonal allergic rhinitis who were 25 to 44 years of age. Intervention: One dose of fexofenadine (60 mg), diphenhydramine (50 mg), al cohol (approximately 0.1 % blood alcohol concentration), or placebo, given at weekly intervals before participants drove for 1 hour in the lowa Drivin g Simulator. Measurements: The primary end point was coherence, a continuous measure of participants' ability to match the varying speed of a vehicle that they wer e following. Secondary end points were drowsiness and other driving measure s, including lane keeping and response to a vehicle that unexpectedly block ed the lane ahead. Results: Participants had significantly better coherence after taking alcoh ol or fexofenadine than after taking diphenhydramine. Lane keeping (steerin g instability and crossing the center line) was impaired after alcohol and diphenhydramine use compared with fexofenadine use. Mean response time to t he blocking vehicle was slowest after alcohol use (2.21 seconds) compared w ith fexofenadine use (1.95 seconds). Self-reported drowsiness did not predi ct lack of coherence and was weakly associated with minimum following dista nce, steering instability, and left-lane excursion. Conclusions: Participants had similar performance when treated with fexofen adine or placebo. After alcohol use, participants performed the primary tas k well but not the secondary tasks; as a result, overall driving performanc e was poorer. After participants took diphenhydramine, driving performance was poorest, indicating that diphenhydramine had a greater impact on drivin g than alcohol did. Drowsiness ratings were not a good predictor of impairm ent, suggesting that drivers cannot use drowsiness to indicate when they sh ould not drive.