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AND THE
ANSWER IS . . .
The early H1 antihistamines such as diphenhydramine (Benadryl) can be effective in reducing some of the symptoms of seasonal allergic rhinitis. However, because these agents are quite lipophilic, they pass readily into the central nervous system (CNS). Possibly because histamine can be a neurotransmitter in the CNS, use of these antihistamines was associated with a high incidence of sedation and other cognitive effects. In careful blinded studies by Weiler et al (1) using a driving simulator showed that prior diphendydramine ingestion adversely effected reaction time, coordination and other aspects of driving performance in a number of subjects to the degree seen with ingestion of moderate doses of ethanol (sufficient to render blood alcohol levels above 0.08 - 0.1). What was equally important, a number of the diphenhydramine-treated subjects with impaired driving capacity did not "feel drowsy". Therefore, diphenhydramine and other "first generation" antihistamines would be contra-indicated in an interstate truck driver.
In contrast, the incidence of sedation with usual doses of the "non-sedating" antihistamines, fexofenadine (Allegra) and loratadine (Claritin) are quite low (2-3%). In the studies by Weiler et al referred to above (1), there was no significant difference in the driving capacities of subjects pre-treated with fexofenadine vs placebo. A recent conference report of experts in aerospace medicine and allergic rhinitis treatment concluded that usual doses of fexofenadine can be safely recommended for use in individuals involved in skilled activities, such as pilots, without the concern of sedation (2). Studies using other approaches have shown no effect of treatment of loratadine on cognitive functions. The reported incidence of sedation with use of cetirizine (Zyrtec) is somewhat higher (10-14%) with higher incidence than other "non-sedating" antihistamines (3). Therefore I would be somewhat reluctant to prescribe cetirizine for a truck driver driving for long periods at high speed. However, recent studies have suggested that levocetirizine, an analog of cetirizine, does not affect cognitive
function. (4)
References
1. Ann Intern Med 2000 7;132:354-63
2. Curr Med Res Opin 2002;18:332-7
3. Allergy 2002;57:534-7
4. J Allergy Clin Immunol 2003;111:623-7
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