In patients with pre-existing pain for < 6 months , the mean relative change of the numeric pain rating scale score on days 7-14 to week 12 versus baseline was -36.6 % [ 4.6 standard error of the mean ( SEM ) ; n = 105 ] , -25.1 % ( 1.9 SEM ; n = 311 ) in patients with pain duration of 6 months to 2 years , -22.3 % ( 1.6 SEM ; n = 391 ) in patients with pain for > 2-10 years , and -19.2 % ( 2.6 SEM ; n = 99 ) in patients with pain for > 10 years .