The one year placebo and active controlled studies have confirmed efficacy and safety. No evidence of tolerance to the bronchodilator effects of tiotropium has been seen. Consistent improvements of health outcomes were also demonstrated. In the one-year studies, statistically significantly fewer patients in the tiotropium groups experienced exacerbations, or were hospitalised for exacerbations. Additionally, time to first exacerbation was increased. This suggests that moderate and severe exacerbations are reduced in-patients treated with tiotropium. The mechanism underlying this is not known, but may be associated with sustained airway opening.