METHODS: Eight outpatients (aged 20-70 yrs) with chronic cough, i. e a cough lasting for longer than 8 weeks, were studied according to an open- label, placebo controlled, cross-over design. On each different study days, at least 48 hours apart, each participant to the study were randomly administered a single dose of Levodropropizine (Levotuss ® 6% drops; 60 mg = 20 drops) or Codeine (Codeine 15 mg = 45 drops) or matching placebo (45 drops of a multivitamin compound). Subsequently, participants performed the Reid CO2 re-breathing test. With this technique, patients breathed, for 5 min, through a 2-way valve connected to a rebreathing bag filled with a mixture of 93% Oxygen and 7% CO2. During the test fractional end tidal CO2 (FETCO2) and inspiratory minute ventilation (I) were continuously monitored. With the rebreathing method, the CO2 sensitivity was calculated as the slope of the relationship between I and FETCO2.