Many different brands and forms of oral antihistamines are available over the counter (OTC).
Oral antihistamines are available as pills, chewable tablets, orally disintegrating tablets, capsules, and liquid.
Medicinal chemistry was very different in those days compared with the present day as elegantly described by Emanuel in his review entitled "Histamine and the antiallergic antihistamines: a history of their discoveries." The usual way of testing novel compounds was to measure histamine-induced contractions of pieces of muscle from experimental animals, usually guinea-pig intestine, suspended in an organ bath.
Candidate antihistaminic compounds were primarily modifications of those synthesized as cholinergic antagonists and are from diverse chemical entities, ethanolamines, ethylene diamines, alkylamines, piperazines, piperidines, and phenothiazines.
© 2011 World Allergy Organization; licensee Bio Med Central Ltd.This is an Open Access article distributed under the terms of the Creative Commons Attribution License ( which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Second, they have unwanted side effects, particularly central nervous system and anti-cholinergic effects, and have the potential for causing severe toxic reactions which are not shared by second-generation H.