Coughing up sputum that is acidic may be caused by a lung infection or GERD. The first thing to consider when coughing up sputum that is acidic is gastroesophageal reflux. Gastric juices reflux through the esophagus into the throat or even the mouth and mix with the phlegm. This is actually a mixture of gastric juices and phlegm, and the symptoms may disappear after the GERD is controlled. In addition, it can also be caused by lung infection. Bacterial, viral and fungal infections in the lungs can increase the amount of phlegm. However, the sputum cannot be expelled completely and can become acidic when it collects in the lungs for a longer period of time, and the sputum itself can also become acidic due to the infection. The exact cause of the disease should be clarified after medical examination and treatment should be carried out according to the doctor’s instructions.