Try not to take phlegm removing drugs before the lung function test, otherwise it will easily affect the test results. Lung function test is mainly used to assess the lung function of some respiratory system diseases, if the lung function test is conducted after taking phlegm removing drugs, it may affect the values of some lung function indexes and interfere with the diagnostic judgment of the disease. Lung function test has a high diagnostic value for airway pathology and lung diseases, and it is also necessary to avoid intake of alcohol, caffeine, hormone drugs, etc. and avoid smoking. In addition, patients should avoid strenuous activity thirty minutes prior to the pulmonary function test. Patients with possible respiratory disease should seek medical attention and have a pulmonary function test performed as prescribed by their physician.