IOWA CITY, Iowa -- A medication commonly found in over-the-counter cold and allergy remedies may cause more driving impairment than being legally drunk. University of Iowa researchers made the finding by studying the driving 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. In comparison, performance after taking fexofenadine was comparable to performance after taking the placebo, an inactive substance. The UI investigators were the first to compare the two medications and alcohol in individuals driving in a high-fidelity driving simulator. The findings will appear 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."

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.

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"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. When taking fexofenadine or the placebo, participants matched the speed of the lead car and followed it at a safer distance than when they received alcohol.

Subjects had greater trouble steering and crossed the centerline more frequently after alcohol and diphenhydramine use. However, they did not experience such impairment after taking fexofenadine.

Weiler said this second-generation antihistamine does not appear to cause impairment in driving performance perhaps because, unlike diphenhydramine, it does not pass the blood-brain barrier, which otherwise may cause drowsiness and impairment.

"Overall, our observations suggest that the second-generation antihistamine is less impairing than alcohol or the first-generation antihistamine," Weiler said.

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He added that the Iowa Driving Simulator (IDS) allowed the investigators to examine performance in a manner not possible with lower fidelity driving simulators or with on-the-road driving.

The National Advanced Driving Simulator, the world's most advanced driving simulator, will be housed at the UI and replace the IDS in July.

The UI study was supported in part by grants from the National Institutes of Health and from Hoechst Marion Roussel, Inc., Kansas City, Mo. The company is now called Aventis Pharmaceuticals, Inc., and manufactures Allegra.

(Editors: To receive a copy of the journal article mentioned in the release, contact the American College of Physicians -American Society of Internal Medicine at (215) 351-2656.)

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