A method of treating and/or preventing allergic and inflammatory conditions of the skin or upper and lower airway passages, e.g. seasonal allergic rhinitis, perennial allergic rhinitis, or chronic idopathic urticaria, in a human more 12 years old, by administering an amount of desloratadine, e.g. 2×2.5 mg or 5 mg/day for a time sufficient to produce a geometric mean steady state maximum plasma concentration of desloratadine in the range of about 2.90 ng/mL to about 4.54 ng/mL, or a arithmetic mean steady state maximum plasma concentration of desloratadine in the range of about 3.2 ng/mL to about 5.0 ng/mL is disclosed.