New insights into the second generation antihistamines

Second generation antihistamines are recognised as being highly effective t reatments for allergy-based disease and are among the most frequently presc ribed and safest drugs in the world. However, consideration of the therapeu tic index or the benefit/risk ratio of the H-1 receptor antagonists is of p aramount importance when prescribing this class of compounds as they are us ed to treat non-life threatening conditions. Then are many second generatio n antihistamines available and at first examination these appear to be comp arable in terms of safety and efficacy. However, the newer antihistamines i n fact represent a hererogeneous group of compounds, having markedly differ ing chemical structures, adverse effects, half-life, tissue distribution an d metabolism, spectrum of antihistaminic properties, and varying degrees of anti-inflammatory effects. With regard to the latter, there is growing awa reness that some of these compounds might represent useful adjunct medicati ons in asthma therapy. In terms of safety issues, the current second genera tion grouping includes compounds with proven cardiotoxic effects and others with the potential for adverse drug interactions, Moreover, some of the se cond generation H1 antagonists have given cause for concern regarding their potential to cause a degree of somnolence in some individuals. It can be a rgued, therefore, that the present second generation grouping is too large and indistinct since this was based primarily on the concept of separating the first generation sedating compounds from nonsedating H1 antagonists. Al though it is too early to talk about a third generation grouping of antihis tamines, future membership of such a classification could be based on a low volume of distribution coupled with a lack of sedating effects, drug inter actions and cardiotoxicity.