Some good over-the-counter antihistamine medicines include Claritin, Zyrtec, Benadryl and Allegra, according to WebMD. Some good over-the-counter decongestants include Afrin nasal spray, Sudafed and Neo-Synephrine.
Antihistamines help to treat allergies by preventing a chemical called histamine from attaching to receptors in a person's blood vessels, states WebMD. Histamine is the chemical that causes parts of the body to itch or swell. Some older antihistamines may cause drowsiness, but most second and third-generation antihistamines do not.
Decongestants work by shrinking blood vessels and nasal tissues. This helps ease the swelling and excess mucus secretions that sometimes accompany an allergic reaction, according to WebMD.