Antihistamines worse than alcohol for driving

A medication commonly found in over-the-counter cold and allergy remedies may cause more driving impairment than being legally drunk, according to University of Iowa (UI) researchers.

The researchers studied the performance of people who had hay fever and were given diphenhydramine (Benadryl), fexofenadine (Allegra), alcohol and a placebo.

The subjects' performance, tested in the Iowa Driving Simulator, was poorest after taking diphenhydramine -- even poorer than when they were legally drunk.

The UI investigators were the first to compare the two medications and alcohol in individuals driving in a high-fidelity driving simulator. The findings appeared in the March 7 issue of the Annals of Internal Medicine.

"First-generation antihistamines, such as diphenhydramine, are known to affect driving performance. However, we were surprised to find that this antihistamine has more impact on driving performance than alcohol does," said John M. Weiler, M.D., UI professor of internal medicine and the study's lead author. "In contrast, we found that fexofenadine, or Allegra, a second-generation antihistamine, did not impair driving performance."

Dr. Weiler added that participants could not predict their driving impairment based on how drowsy they felt.

"Drowsiness was only weakly associated with minimum following distance, steering instability and crossing into the left lane," he said. "These results suggest that people should carefully read warning labels on all medications. Even if you do not feel drowsy after taking an antihistamine or alcohol, you may be impaired."

The investigators studied 40 licensed drivers, ages 25 to 44, who had hay fever -- allergies to ragweed pollen -- and who had previously used antihistamines to treat the condition. Allergic rhinitis affects more than 39 million people in the United States.

"It is not commonly recognized that more than half of the states have laws prohibiting driving under the influence of sedating medications," Weiler said.

The researchers tested the participants' ability to follow a lead car that changed its speed randomly. Alcohol-treated subjects performed this task well, but drove closer to the car and had less steering control.

Weiler noted that previous studies have shown that drunk drivers may perform one task well but at the expense of other crucial driving tasks.