RT Journal Article SR Electronic T1 Fundamental problem with opioid trials for chronic pain JF Canadian Medical Association Journal JO CMAJ FD Canadian Medical Association SP 1307 OP 1308 DO 10.1503/cmaj.1060140 VO 176 IS 9 A1 Tushar Mehta YR 2007 UL http://www.cmaj.ca/content/176/9/1307.3.abstract