Kumar, A., K. Gupta, P. K. Gupta, B. Rastogi, B. Agrawal, and P. Kalra. “Clinical Efficacy of Nalbuphine Versus Tramadol As Analgesic Adjuvant to Fentanyl During Major Abdominal Surgery Performed Under General Anesthesia- A Double Blind Randomized Study”. Academia Anesthesiologica International, Vol. 4, no. 1, May 2019, pp. 76-80, doi:10.21276/aan.2019.4.1.17.