Among 221 evaluable participants , 80 ( 79 % ; 95 % CI , 70 % -88 % ) assigned to receive tadalafil retained erectile function between weeks 28 and 30 compared with 61 ( 74 % ; 95 % CI , 63 % -85 % ) assigned to receive placebo ( P = .49 ) ; an absolute difference of 5 % ( 95 % CI , -9 % to 19 % ) .