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My 2.5 year old son has had allergy problems from approximately six weeks after birth. It seems to have improved with time. Recently, he started coughing seriously, especially at night. None of the medication seems to work. Is there anything I could give him to relieve him of this coughing? He sometimes vomits after those coughs.
Sometimes the "trigger" (allergen) which stimulates the allergic response can change - certainly if pollen/grasses are a problem, the natural weather pattern may be a problem. Dust may build up with time. You obviously can't change the weather but check to make sure your furnace/air conditioning filters are clean.
Another reason for a breakthrough of symptoms is that often, after a few months, the medication which worked before, loses its effectiveness (temporarily) and you have to use another. After a while you go back to medicine #1 when #2 loses its effectiveness.