Antihistamines in rhinoconjunctivitis.

In allergic rhinoconjunctivitis, histamine is known to contribute predominantly to nasal itch, sneeze, rhinorrhea, conjunctival itch, and lacrimation and these symptoms benefit most from H1-antihistamine therapy. The discovery in the early 1980s of nonsedating H1-receptor antagonists contributed dramatically to the more widespread acceptance of this mode of therapy. This also led to the undertaking of well-designed clinical trials that have added significantly to our understanding of allergic rhinitis. Oral treatment modifies both nasal and ocular symptoms and provides effective control throughout a 24-h period with once- or twice-daily medication. The advent of topical H1-receptor antagonists offers a wider choice of treatments and provides equal or greater efficacy with lower systemic bioavailability. While having a major impact on rhinoconjunctivitis symptoms, H1-antihistamines do not fully modify disease since histamine is not the only contributor to symptom generation in allergic rhinoconjunctivitis. While the search for oral H1-antihistamines with more widespread "antiallergic" activity continues, the currently available medications modify predominantly histamine-regulated events despite in vitro evidence of greater potential. The development of these new medications may be the next significant advance in this mode of treatment.