Onset of action of astemizole

Astemizole has been described to have a slow onset of action and this has to a large extent been attributed to its unusual pharmacokinetic profile. Yet, pharmacokinetically, there are no reasons why astemizole should not act within the first hours after intake, since plasma levels of unchanged astemizole are maximal within 40 min and there is fast tissue distribution. Animal pharmacology data show effective antihistamine activity with astemizole within 1 h after intake. Clinical data referring to the onset of action of astemizole with regard to symptom relief were available from 27 studies on over 7000 patients. These studies showed astemizole to provide symptom relief within 4 to 6 h of intake in 16-85% of patients, and within 24 h in 42-90% of patients; these figures are comparable to those reported for other new antihistamines. Comparative studies between astemizole and other new antihistamines (terfenadine, loratadine and cetirizine) indicated no or only minor differences in onset of clinical effect. Two recent studies compared astemizole to both terfenadine and loratadine under well-controlled circumstances. One was a pollen challenge study showing all three drugs to be able to reverse the challenge-induced effects within 1-3 h after intake. In the other, the mean time to relief of at least one rhinitis symptom was assessed as 18 min for astemizole, 24 min for terfenadine and 36 min for loratadine. Compared with the systemic decongestant pseudoephedrine, time to first relief of symptoms was similar for astemizole (4 h) and pseudoephedrine (3.5 h). Controlled clinical trials thus show astemizole to provide fast symptom relief. This was confirmed in patient surveys in Canada and Switzerland: over 80% of patients were very satisfied with astemizole and experienced onset of action within hours. In conclusion, astemizole results in symptom relief within hours after its administration. Its onset of action is not different from that of other nonsedating antihistamines and is also similar to that of pseudoephedrine.