Common causes of constant coughing and excess phlegm include postnasal drip, asthma and gastroesophageal reflux disease, according to Mayo Clinic. More serious causes include viral and bacterial infections, chronic bronchitis, and the side effects of blood pressure drugs.
Postnasal drip occurs when the sinuses or nose produce excess mucus or phlegm that drips down the back of the throat and causes people to cough, as Mayo Clinic explains. An asthma-induced cough commonly occurs after an upper respiratory tract infection or when asthma patients are exposed to chemicals, fragrances or cold air. People with gastroesophageal reflux disease experience stomach acid flowing back into the tube that connects the throat to the stomach. The irritation often leads to constant coughing and the production of excess phlegm.
People with infections such as pneumonia, influenza, colds and upper respiratory tract infections may experience a chronic cough and excess phlegm, according to Mayo Clinic. Chronic coughing is also a symptom of bronchitis, a condition in which inflammation blocks the major airways. Certain blood pressure drugs, such as angiotensin-converting enzyme inhibitors, can also cause chronic coughing.
Individuals with infectious diseases, cystic fibrosis, COPD, lung cancer and aspiration problems often experience constant coughing or excess phlegm as symptoms of such conditions, according to Mayo Clinic.