Comparative tolerability of second generation antihistamines

Second generation histamine H-1 receptor antagonists, the so-called 'nonsed ating' antihistamines, have high potency and additional antiallergic proper ties as well as H-1 antagonism and are associated with fewer adverse effect s compared with the first generation antihistamines, A number of drugs in t his class are approved for use: acrivastine, astemizole, azelastine, cetiri zine, ebastine, fexofenadine, loratadine, mizolastine and terfenadine, All of them have a more favourable risk-benefit ratio with regard to the CNS ad verse effects, Even those second generation antihistamines that are not act ually 'nonsedating' are less impairing than their predecessors, but not one of them is entirely devoid of CNS activity. Under certain circumstances some antihistamines may affect cardiac repolari sation resulting in cardiovascular adverse effects. Serious cardiovascular effects have been reported with terfenadine and astemizole when they are us ed in high dosages or when they are given to 'at risk' patients. Animal mod els indicate that there might be a potential risk of cardiovascular adverse effects with other antihistamines as well. However, up to now there is no clinical evidence for this assumption, despite some confusing reports. Like wise there has been much discussion about a link between these agents and c arcinogenicity. However, there is no evidence that any of the second genera tion antihistamines increase the risk of tumour growth in humans. Small children, elderly patients and persons with chronic renal or liver im pairment are special groups in which the individual adverse effects of the second generation antihistamines must be kept in mind. The dosage for an in dividual has to be modified with respect to their metabolic situation. Despite the fact that some of the second generation antihistamines are list ed in the US Food and Drug Administration pregnancy risk classification as class B, the use of second generation antihistamines should be avoided duri ng pregnancy and they should never be administered to nursing mothers. Taking into account their negligible CNS activity, the low incidence of car diovascular adverse effects, their lack of anticholinergic effects and othe r benefits, this class of antiallergic drugs represents a definite advance in therapy.