Driving and Antihistamines
abstract & commentary
Source: Weiler JM, et al. Effects of fexofenadine, diphenhydramine, and alcohol on driving performance. Ann Intern Med 2000;132:354-363.
The objective of this trial was to compare the effects of fexofenadine (60 mg), diphenhydramine (50 mg), alcohol (approximately 0.1% blood concentration), and placebo on various measures of driving performance.
A randomized, double-blind, double-dummy, crossover design was used so that each participant received all four treatments on four successive sessions at weekly intervals in a driving simulator. The participants were 40 licensed drivers with seasonal allergic rhinitis, ages 25 to 44 years, including 15 men and 25 women.
The primary outcome measure was coherence, a measure of a participant’s ability to maintain a constant distance from a lead car that varied its speed randomly. Secondary outcome measures were self-reported drowsiness and other driving end points, including lane-keeping, minimum following distance, steering instability, and response to an unexpected blocking vehicle.
Pairwise comparisons revealed that the diphenhydramine group performed car-following with significantly less coherence (0.877 ± 0.019 CI, 0.837-0.911) than the alcohol group (0.920 ± 0.014 CI, 0.891-0.945), fexofenadine group (0.915 ± 0.014 CI, 0.884-0.940), or the placebo group (0.906 ± 0.015 CI, 0.875-0.933). Lanekeeping was impaired after alcohol and diphenhydramine use compared with fexofenadine and placebo.
After consuming alcohol, participants performed car-following at significantly smaller minimum following distances (15.1 m) than they did after taking fexofenadine (17.1 m) or placebo (17.4 m). Self-reported drowsiness did not predict lack of coherence and was weakly associated with the other secondary end points. Comment by Stephanie B. Abbuhl, MD, FACEP
One could argue that the statistically significant differences in the outcome measures of this study do not necessarily translate into clinically significant differences. It is possible that impairment of coherence in a driving simulator does not predict poor driving performance in real life. In fact, it is not entirely clear why coherence was chosen as the primary outcome measure; an accompanying editorial points out that coherence has not been validated as a true indicator of risk for motor vehicle crashes (MVC).1
On the other hand, coherence and the other secondary measures may actually predict a real risk of MVC. Given that we do not have good studies to answer the real-life question, and considering that multiple other studies have shown that sedating antihistamines impair psychomotor performance, it behooves us to prescribe a nonsedating antihistamine for patients who must drive or operate machinery/equipment. If cost prohibits prescribing the nonsedating antihistamines, then strict warnings must accompany discharge instructions so that patients avoid driving or any other task that involves sharp psychomotor performance. It is disturbing that most of the 39 million persons in the United States with allergic rhinitis take over-the-counter medications, which generally contain a first-generation antihistamine.
Reference
1. Hennessy S, Strom BL. Nonsedating antihistamines should be preferred over sedating antihistamines in patients who drive. Ann Intern Med 2000;132:405-407 (editorial).
30. In the study comparing the effects of fexofenadine, diphenhydramine, alcohol, and placebo on driving performance, all of the following are true except:
a. Study participants had significantly better coherence (a measure of driving performance) after taking fexofenadine than after diphenhydramine.
b. Self-reported drowsiness predicted lack of coherence.
c. Participants under the influence of alcohol did surprisingly well on the coherence outcome measure, but they kept a shorter distance to the car they were following and had less control over lane-keeping.
d. Coherence is one of many measures of driving performance in a driving simulator and has not been validated as a true indicator of risk for motor vehicle crashes.
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