Nasal spray for hay fever. Illustration of a nasal spray being used to deliver a corticosteroid drug into the nose to treat hay fever. Hay fever is caused by an allergic reaction to proteins on the surface of airborne pollen or dust particles. The mucous membrane inside the nose becomes inflamed, resulting in sneezing, congestion and a watery discharge. Corticosteroid drugs delivered by spray reduce inflammation by suppressing the action of both T-lymphocyte white blood cells and mast cells. As the drug is delivered directly onto the inflamed tissue it works quickly and causes fewer side-effects than oral medication.