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First-Generation Antihistamines

The older, so-called first generation antihistamines include:Diphenhydramine (Benadryl). Carbinoxamine (Clistin) Clemastine (Tavist) ,.Chlorpheniramine (Chlor-Trimeton). Some health professionals recommend this agent if antihistamines are required during pregnancy. Îœany believe it is as effective as the second generation antihistamines and much less expensive Brompheniramine (Dimetane). First-generation antihistamines contain anticholinergics, which tend to produce the side effects that differentiate this group from second-generation antihistamines. Side Effects. Drowsiness and impaired thinking. Dry mouth. Dizziness.

Agitation. Insomnia or nightmares. Sore throat. Rapid heart beat and chest tightness (uncommon and should be reported). Men with enlarged prostate glands may experience difficulty urinating. Drowsiness is the most distressing side effect reported from first-generation antihistamines, and is potentially serious.

Some evidence indicates this effect poses a higher than average risk for work-related and automobile accidents than alcohol, narcotics, or prescription sedatives. Of interest, however, was a 2001 study suggesting that first-generation antihistamines do not have the same effect on children. In the study, children who took Benadryl had no greater impairment of alertness or learning than children who took loratadine (Claritin), one of the common second-generation antihistamines, which are supposed to cause less sleepiness. In fact, a major 2003 analysis could find no strong differences in effects on sedation in general between the first- and second-generation antihistamines. Still, experts caution against the first-generation antihistamines for people most at risk from sedative effects, particularly elderly individuals.

The sedating effect of first generation H(1)-antihistamines has been associated with their ability to penetrate the blood-brain barrier (BBB) and lack of efflux by P-glycoprotein (Pgp). Second generation H(1)-antihistamines are relatively free of sedation and their limited brain penetration has been suggested to arise from Pgp-mediated efflux.

References

Obradovic T, Dobson GG, Shingaki T, Kungu T, Hidalgo IJ. Assessment of the first and second generation antihistamines brain penetration and role of P-glycoprotein. Pharm Res. 2007 Feb;24(2):318-27. Epub 2006 Dec 19.