Buynak R, Rappaport SA, Rod K, et al. Long-term Safety and Efficacy of Tapentadol Extended Release Following up to 2 Years of Treatment in Patients With Moderate to Severe, Chronic Pain: Results of an Open-label Extension Trial. Clin Ther. 2015;37(11):2420-2438. doi:10.1016/j.clinthera.2015.08.014