Longstanding or repetitive exposure to environmental allergens creates chronic allergic inflammation - most commonly of the membranes of the nasal passages, eyes and throat - that in turn produces the classic chronic symptoms of allergy: congestion, sniffling, runny nose, post-nasal drip, throat-clearing, cough, sneeze and itchiness.
Thus, chronic allergy sufferers usually require medication daily or twice-daily to control their symptoms, so long as they are continually exposed to their allergens.