Cough is a normal physiological defense reflex, and it is the only way for the human body to clear mucus from the respiratory tract. Children under the age of 3 have a poor cough reflex, making it difficult to expel phlegm. If parents give them strong cough suppressants as soon as they cough, the cough may stop temporarily, but the phlegm cannot be smoothly expelled and instead accumulates in large amounts in the trachea and bronchi, which can cause tracheal obstruction.