Compared with placebo , erythromycin reduced the rate of pulmonary exacerbations over the 48 weeks of the study in patients with P. aeruginosa-dominated infection ( median 1 [ IQR 0-3 ] vs 3 [ 2-5 ] ; median difference -2 , 95 % CI -4 to -1 ; p = 001 ) , but not in those without P. aeruginosa-dominated infection ( 1 [ 0-2 ] vs 1 [ 0-3 ] ; median difference 0 , -1 to 0 ; p = 041 ; interaction p = 004 ) .