Histamine is the most potent mediator that is released during the early phase of an allergic reaction and causes itching, sneezing, rhinorrhoea, and nasal congestion. The Allergic Rhinitis and its Impact on Asthma (ARIA) guidelines recommend using second-generation oral antihistamines for both seasonal (SAR) and perennial (PAR) rhinitis; these molecules represent the most suitable preparations to treat allergic rhinitis because of their already mentioned ‘antiallergic-anti-inflammatory’ activity and excellent safety profile. Such drugs can be used as needed if the symptoms are occasional. In SAR, treatment with anti-H1 antihistamines could be started before exposure to the allergens to downregulate constitutive H1 receptor activity and shift the equilibrium from the active form of the H1 receptor to the inactive form, thus preventing the massive release of histamine by mast cells; this treatment has to be continued for the entire duration of pollination. In PAR, the treatment should instead be modulated based on clinical symptoms, and it has the dual purpose of controlling the persistent mucosal inflammation that reduces the inflammatory mucosal infiltrate and adhesion molecule expression [
16,
17].
Among antihistamines, cetirizine has a relatively higher affinity and selectivity for H1 receptors, which confers a more potent, faster onset, and longer duration of action with even anti-inflammatory properties independent of its anti-H1 effects [
18,
19]. Ciprandi et al. [
20] demonstrated that cetirizine exerts a significant anti-inflammatory activity in children with PAR decreasing the nasal levels of interleukin (IL)-4 and IL-8. Similar results were obtained by Uguz and colleagues [
21], who demonstrated that cetirizine induces a shift in the Th1/Th2 cytokine balance towards a Th1 response, increasing the production of interferon (IFN)-gamma and IL-10. From a clinical point of view, cetirizine was shown to be more effective when compared with placebo in the treatment of SAR with a significant reduction in the number of days characterized by the absence of symptoms assessed by questionnaires [
14,
22,
23]. Furthermore, the efficacy of cetirizine appears to be immediate (within 24 h) and lasting over time when compared with first-generation antihistamines and is not burdened by significant side effects [
24]. Also, when compared with other second-generation antihistamines (see Table
1), cetirizine seems to be more effective: Lee et al. showed that the 12-week treatment program with cetirizine was more effective than levocetirizine [
25], and recently, in a randomized, placebo-controlled study, a better effect of cetirizine was found in the treatment of allergic rhinitis when compared with loratadine [
26]. Concerning PAR, the effect of cetirizine has also been compared to montelukast. Although both products appear to be effective in reducing symptoms compared to placebo, cetirizine offers better results in improving nasal itching [
27]. Furthermore, clinical studies demonstrate the importance of long-term cetirizine therapy: in children with dust mite allergy, treatment for at least 6 months results in a significant reduction in the prescription of other drugs (e.g., inhaled corticosteroids, beta2-agonists, antibiotics) compared to placebo [
28]. In the same category of children, it has been shown that treatment for at least 3 years with cetirizine is associated with a reduction of new allergic sensitizations [
29].
Regarding dosage, it was confirmed that 10 mg per day in children over 6 years (divided into two administrations if less than 12 years) is better when compared with 5 mg in terms of symptom reduction, both concerning rhinitis and conjunctivitis. The administration of 5 mg is only useful for the reduction of sneezing [
30]. Table
2 summarizes the evidences on the efficacy of cetirizine in the treatment of allergic rhinitis and conjunctivitis.
Table 2
Summary of evidence on the efficacy of cetirizine in the treatment of allergic rhinitis
| Perennial allergic rhinitis | 20 children | Decreasing of nasal IL-4 and IL-8 levels |
| Perennial allergic rhinitis | 13 children | Increasing the production of IFN-gamma and IL-10 |
| Seasonal allergic rhinitis | 107 children aged 2–6 years | Improvement in sneezing, rhinorrhea, nasal obstruction, nasal and ocular pruritus than placebo |
| Seasonal allergic rhinitis | 124 children aged 6–12 years | Improvement in sneezing, rhinorrhea, nasal obstruction, nasal and ocular pruritus than placebo |
| Seasonal allergic rhinitis | 209 children aged 6–11 years | Improvement in sneezing, nasal discharge, itchy eyes, itchy nose or mouth, conjunctivitis, nasal congestion than placebo |
| Seasonal allergic rhinitis | 683 children aged 6–11 years | Improvement in TSSC than loratadine and placebo |
| Perennial allergic rhinitis | 74 children aged 6–12 years | Improvement in TSS than levocetirizine and placebo |
| Perennial allergic rhinitis | 60 children aged 2–6 years | Improvement in eosinophil percentage in nasal smears, PRQLQ and TSS. Cetirizine better than montelukast for itching |
| Rhinitis and/or mild asthma | 10 children | Improvement in symptoms and reduction of drug consumption than placebo |
| Mite allergy | 20 children | Lower incidence of new sensitisations |
| Seasonal allergic rhinitis | 209 children aged 6–11 years | Improvement in TSS than placebo |