The cohort randomized to inhaled tobramycin and reporting azithromycin use showed a significant decrease in the percent predicted FEV1 after one and three courses of inhaled tobramycin when compared with those not reporting azithromycin use ( 28 d : -0.51 vs. 3.43 % , P < 0.01 ; 140 d : -1.87 vs. 6.07 % , P < 0.01 ) .