Antihistamines: Astelin (Azelastine nasal spray) Zyrtec (Ceterizine) Chlor-Trimeton Allergy, Efidac 24, (Chlorpheniramine) Tavist (Clemastine) Clarinex (Desloratadine) Benadryl (Diphenhydramine) Allegra (Fexofenadine) Claritin, Alavert (Loratadine) Phenergan (Promethazine) Although antihistamines are not very effective at treating nasal congestion, they are generally the most effective defense against symptoms of sneezing, itching, and runny nose. This is why antihistamines are often combined with decongestants in medications. As previously mentioned, antihistamines block the production of histamine, thereby reducing the effects of swollen nasal passages and runny nose. The antihistamine prevents the histamines from binding to the histamine receptors which inhibits the reaction. The majority of antihistamines are taken orally. Generally they need to be taken at least twice a day to be effective. Antihistamines are notorious for causing drowsiness in approximately 35% of patients, however continued use can build of a tolerance for this unfortunate side effect. In addition, the newer formulations are much less likely to cause excessive drowsiness than the older medications. Other side effects of antihistamines may include blurry vision, dry eyes, constipation, urine retention, and pulse awareness. The most common side effects vary with the different types of antihistamines.