In patients with pre-existing pain for < @ months , the mean relative change of the numeric pain rating scale score on days @-@ to week @ versus baseline was -@ % -LSB- @ standard error of the mean ( SEM ) ; n = @ -RSB- , -@ % ( @ SEM ; n = @ ) in patients with pain duration of @ months to @ years , -@ % ( @ SEM ; n = @ ) in patients with pain for > @-@ years , and -@ % ( @ SEM ; n = @ ) in patients with pain for > @ years .