For high-risk patients ( n = @ ) , high-dose NAC compared with placebo significantly reduced exacerbation frequency ( @ vs @ -LSB- P = @ -RSB- and @ vs @ -LSB- P = @ -RSB- at @ and @ months , respectively ) , prolonged time to first exacerbation ( P = @ ) , and increased the probability of being exacerbation free at @ year ( @ % vs @ % , P = @ ) .