Open-label, single-center, randomized and parallel-group study to compare the efficacy of olopatadine hydrochloride and dextromethorphan hydrobromide in prolonged and chronic cough

INTERVENTION: olopatadine hydrochloride 5mg b.i.d, for 2 weeks dextromethorphan hydrobromide 15mg t.i.d, for 2 weeks CONDITION: Prolonged and chronic cough PRIMARY OUTCOME: Symptom scores and VAS score(at 0 and 2 weeks); Capsaicin cough sensitivity test(at 0 and 2weeks); Concentrations of substance P in sputum and plasma(at 0 and 2weeks) SECONDARY OUTCOME: 1. Pulmonary function tests(at 0 and 2weeks); 2. Inflammatory biomarkers of induced sputum and serum(at 0 and 2weeks); 3. Exhaled nitric oxide concentration(at 0 and 2weeks); 4. Parameters of impulse oscillometry(at 0 and 2weeks); 5. safety INCLUSION CRITERIA: 1. Cough persisting for longer than 3 weeks. 2. No radiological evidence of chest disease. 3. Who gave written informed consent.