If too much mucus  accumulates in the back of the nasal cavity or mouth, a postnasal drip  occurs. This normally causes a feeling of a “tickling” or itchy throat that refuse to go away. The mucus may be transparent and watery or dense with a green, yellow, or white color. Frequent coughing, watery eyes, and a clear throat, typically arise from the pain associated with a post-nasal drip. It can make breathing difficult in individuals with asthma.
Allergies, colds, and influenza are the common causes of post-nasal drip. It also often accompanies other bacterial or viral respiratory infections, or may be caused by simple discomfort, such as consuming very spicy foods. Some drugs, including those containing estrogen, can trigger a post-nasal drip. A post-nasal drip may accompany other symptoms , depending on the underlying disorder.
Post-nasal drip Respiratory symptoms
It can accompany other symptoms that affect the respiratory system, including:
Cough  is a common symptom of varying significance, it is typically a reflex response to stimuli that irritate laryngeal, tracheal or large bronchial receptors. Other causes are inflammation of the respiratory mucosa, and pressure or tension in the air passages, caused by a tumor or enlarged per bronchial lymph nodes. Its presence means a typical airway problem, but the symptom may also be of cardiovascular origin.
Coughing up clear, yellow, light brown, or green mucus is in the case where the post nasal drip is due to sinusitis. This is also regarded as an inflammation of the sinus. This disorder may be brought about by a virus, allergy , or even bacteria. You may experience a sputum of green or yellow  phlegm, postnasal drip, nasal congestion, and pressure in your sinus cavities when it is caused by bacteria.