Randomized Controlled Trial
Analgesic efficacy and safety of tramadol/ acetaminophen combination tablets (Ultracet) in treatment of chronic low back pain: a multicenter, outpatient, randomized, double blind, placebo controlled trial.
J Rheumatol. 2004 Dec 1;31(12):2454-63.
Randomized Controlled Trial
Efficacy and safety of oral tapentadol extended release in Japanese and Korean patients with moderate to severe, chronic malignant tumor-related pain.
Curr Med Res Opin. 2013 Oct 1; 29 (10): 139914091399-409.
Randomized Controlled Trial
Once-daily OROS hydromorphone ER compared with placebo in opioid-tolerant patients with chronic low back pain.
Curr Med Res Opin. 2010 Jun 1;26(6):1505-18.