Antihistamines do not destroy released histamine, nor do they interfere with histamine already bound to the receptor site. Instead, they block the receptor site and subsequently block the histamine from binding to the receptor.
Antihistamines fall into various categories. The first antihistamines that we developed were strongly sedating. These are called first - generation. Second-generation antihistamines were developed later, and are nonsedating.
Oral antihistamines are absorbed into the bloodstream and usually start to work within 60 minutes. They are most effective in the treatment of the following symptoms of allergic rhinitis: sneezing, nasal and
ocular itching, and runny nose. Antihistamines have little or no effect on nasal congestion. That is why they are often prescribed in combination with a decongestant. However, although I believe that long-term use of antihistamines is advised when indicated, I believe that long-term use of decongestants should not be recommended because of their stimulant properties.
Side effects of antihistamines include sedation; dryness in the mouth, eyes, and throughout the body; gastrointestinal distress, cardiac arrhythmias, and prostatic hypertrophy.
Cetirizine (Zyrtec) Diphenhydramine (Benaedryl)
Chlorpheniramine Fexofenadine (Allegra)
(Chlor-Trimeton) Loratadine (Claritin, Alavert)
Nasal antihistamines are considered to be similar in efficacy to oral antihistamines. Nosebleeds are a very rare side effect; a bitter taste can occur, and this can be minimized by tilting your head down. It can, but rarely does, cause, nausea. One type is azelastine (Astelin Nasal Spray).