The physiological mechanism of allergic reactions is the same, however, in everyone.Allergens enter the body -- either through ingestion, inhalation or contact with the skin or mucous membranes.All antihistamines work in the same way: by competing with histamine to prevent or reduce the characteristic signs and symptoms of an allergic reaction: swelling, tearing, itching, and increase in bronchial and other secretions.
During an allergic reaction, mast cells release histamine and other substances.
Oral decongestants may cause dizziness, headache, nervousness, fast heartbeat, increased blood pressure, loss of appetite, and sleep problems.
Nasal anticholinergics can cause a bloody or dry nose, nasal congestion, dry mouth and irritated throat, bad taste in the mouth, dizziness, and nausea.
Serious but unlikely side effects include flu-like symptoms.
Nasal decongestants may cause a temporary burning, stinging, or dryness in the nose, a runny nose, and sneezing.