Antihistamines May Impair Driving More than Alcohol

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Antihistamines May Impair Driving More than Alcohol

Allergic rhinitis afflicts more than 39 million people in the United States alone, and nearly five million people take prescription drugs to manage this condition. Many others go without medical treatment or self-treat using over-the-counter medications containing antihistamines.

Evidence suggests that these medications may carry potential risks, including drowsiness and difficulty in performing everyday tasks such as driving.

This study examined automobile driving performance as influenced by alcohol and antihistamines. Forty licensed drivers (25-44 years of age) with seasonal allergic rhinitis were given one of the following to induce an estimated blood alcohol level of 0.10%: fexofenadine (Allegra - 60 mg), dephenhydramine (Benadryl - 50 mg), or alcohol (800 mg per kg of body weight for men, 640 mg/kg for women). One group was also given a placebo. Each substance was administered once a week for five weeks, with driving tests administered each week using the Iowa Driving Simulator.

All tests took place one hour after administration of the substance, with drivers navigating a simulated 45-mile course on a two-lane rural road. Outcome measures included steering instability (deviation of the participant's car from preferred lane position), responses to an unexpected blocking vehicle, and coherence (ability to match the varying speed of the vehicle they were instructed to follow).

The authors conclude that participants did significantly better on the driving simulation after taking fexofenadine compared with diphenhydramine (Benadryl), noting that "...diphenhydramine may have an even greater impact than does alcohol on the complex task of operating an automobile." They also note that drowsiness was a poor predictor of impairment, suggesting that drivers cannot use drowsiness as an indicator of whether or not they should drive.

This study was funded by a grant from the drug company that makes fexofenadine (Allegra).

Weiler JM, Bloomfield JR, Woodworth GG, et al. Effects of fexofenadine, diphenhydramine, and alcohol on driving performance: a randomized, placebo-controlled trial in the Iowa Driving Simulator. Annals of Internal Medicine 2000:132, pp354-63.