METHODS: Animals were assigned to treatment with citric acid inhalation (CA), or citric acid inhalation with codeine pretreated (COD), or saline solution inhalation (SA). Intervention period is up to 6weeks (22 days or 43 days). All animals were challenged with 0.5 M citric acid on the first day and the last day of exposure. Animals were repeatedly challenged with citric acid or saline solution 3 times weekly. Number of cough was counted. 72hr after the last exposure, lung specimens were obtained for pathological analysis.