(drug testing - impairment by drug) "The observations reported here, combined with past reports, indicate that diphenhydramine clearly impairs driving performance .... This study demonstrates that the firstgeneration antihistamine diphenhydramine may have an even greater impact than does alcohol on the complex task of operating an automobile."
Weiler, John M.; Bloomfield, John R.; Woodworth, George G.; Grant, Angela R. ; Layton, Teresa A.; Brown, Timothy L.; McKenzie, David R.; Baker, Thomas W.; and Watson, Ginger S., "Effects of Fexofenadine, Diphenhydramine, and Alcohol on Driving Performance: A Randomized, Placebo-Controlled Trial in the Iowa Driving Simulator," Annals of Internal Medicine (Philadelphia, PA: American College of Physicians, March 2000) Vol. 132, No. 5, p. 362.
http://www.annals.org/content…
