G. J. Vorsanger, PhD, MD, J. Xiang, PhD, T. J. Gana, MD, PhD, M. L. G. Pascual, MD, MPH, and R. R. B. Fleming, MS, “Extended-release tramadol (tramadol ER) in the treatment of chronic low back pain”, J of Opioid Management, vol. 4, no. 2, pp. 87–97, Jan. 2018.