During treatment , percent heavy drinking days ( naltrexone : mean = 21.60 , SD = 16.05 ; placebo : mean = 22.90 , SD = 13.20 ) ( P = .58 ) and percent days abstinent ( naltrexone : mean = 56.60 , SD = 22.52 ; placebo : mean = 62.50 , SD = 15.75 ) ( P = .39 ) did not differ by group .