Antihistamines and Driving Performance

A study published in the Annals of Internal Medicine (March 2000) reports that sedating antihistamines can impair your driving performance at least as much as alcohol. According to the study, seasonal allergies affect more than 39 million people in the US. But only about 12% of allergy sufferers take prescription drugs while the rest of us stay close to the Kleenex box or use non-prescription allergy medications, some of which cause drowsiness and impair the ability to perform complex tasks.

Researchers tested the effect of two nonprescription antihistamines commonly taken for seasonal allergies on driving performance. They tested diphenhydramine (Benadryl) and fexofenadine (Allegra). Benadryl is known to cause drowsiness and Allegra is not. They also tested the effects of a placebo and alcohol.

They used the Iowa Driving Simulator to assess driving performance in 4 sessions of "virtual driving." Fourty ragweed allergy-afflicted licensed drivers aged 25 to 40 participated in the study. Participants were asked to rate their level of drowsiness before and after receiving one of the antihistamines, alcohol, or placebo. Their performance in routine driving in light traffic, maintaining constant distance behind a leading vehicle, maintaining optimal location with respect to the center line and other lane boundaries, and reacting to an oncoming car or a vehicle blocking traffic were measured using the Driving Simulator.

The authors found that study participants performed similarly after taking fexofenadine (Allegra) and a placebo, but that the drowsiness-causing antihistamine diphenhydramine appears to have an even greater impact on driving ability than alcohol does. The drivers who took diphenhydramine strayed out of their lane more often and exhibited poorer steering control. The self -reported level of drowsiness among people taking diphenhydramine was not found to be a good predictor of driving ability as measured by the Simulator. This suggests that people who take antihistamines may not be good at judging the potential impact of the drug on their performance. Even if you don't feel drowsy, your ability to drive safely and react in a timely manner to unexpected events may still be significantly impaired.

Each year, Medical College of Wisconsin physicians care for more than 180,000 patients, representing nearly 500,000 patient visits. Medical College physicians practice at Children's Hospital of Wisconsin, Froedtert Memorial Lutheran Hospital, the Milwaukee VA Medical Center, and many other hospitals and clinics in Milwaukee and southeastern Wisconsin.