The primary outcome is cough symptom score using Leicester cough questionnaire (LCQ). The secondary outcomes are pulmonary function tests (spirometry, impulse oscillometry) and exhaled nitric oxide and quality of life (SF-36). All outcomes are measured at baseline, 2 weeks, and 4 weeks. Bronchoprovocation test with methacholine is performed at baseline and 4 weeks to determine the provocative concentration of methacholine that induces falling of FEV1 >or =20%. Adverse events will be recorded every visit.