The first line of treatment for allergic rhinitis must be identification and avoidance as far as possible of the relevant allergens. This is rarely completely successful and so pharmaceutical treatment is also needed. Antihistamines are effective in allergic rhinitis, which comprises approximately 80% of rhinitis found in children and 30% in adults. Antihistamines antagonize the effect of histamine at the H1 receptor and their major clinical effects are related to this. They are effective against rhinorrhoea, itch and sneezing but have little effect on nasal obstruction. Clinical trials show between 36% and 80% of patients experiencing good to excellent symptom relief (approximately twice that induced by placebo), in seasonal allergic rhinitis; between 55% and 86% of patients experience moderate to excellent symptom relief in perennial allergic rhinitis. Meta-analysis reveals that antihistamines are inferior to corticosteroids in allergic rhinitis therapy. Antihistamines are generally well tolerated, although serious cardiac dysrhythmias have occurred with terfenadine and astemizole. Their place in therapy is probably as sole agent in mild, intermittent rhinitis and in combination with topical corticosteroids in more severe disease not controlled by corticosteroids alone.
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