After 1 year , we noted 497 acute exacerbations in 482 patients in the N-acetylcysteine group who received at least one dose and had at least one assessment visit ( 116 exacerbations per patient-year ) and 641 acute exacerbations in 482 patients in the placebo group ( 149 exacerbations per patient-year ; risk ratio 078 , 95 % CI 067-090 ; p = 00011 ) .