1. Wicks C, Hudlicky T, Rinner U. Morphine alkaloids: History, biology, and synthesis. Alkaloids Chem Biol. 2021;86:145−342.

2. Crush J, Levy N, Knaggs RD, Lobo DN. Misappropriation of the 1986 WHO analgesic ladder: the pitfalls of labelling opioids as weak or strong. Br J Anaesth. 2022;129:137−42.

3. Dowell D, Ragan KR, Jones CM, Baldwin GT, Chou R. CDC clinical practice guideline for prescribing opioids for pain - United States, 2022. MMWR Recomm Rep. 2022;71:1−95.

4. Manchikanti L, Kaye AM, Knezevic NN, et al. Responsible, safe, and effective prescription of opioids for chronic non-cancer pain: American Society of Interventional Pain Physicians (ASIPP) Guidelines. Pain Physician. 2017;20:S3−S92.

5. Chinman M, Gellad WF, McCarthy S, et al. Protocol for evaluating the nationwide implementation of the VA Stratification Tool for Opioid Risk Management (STORM). Implement Sci. 2019;14:5.

6. Sverrisdottir E, Lund TM, Olesen AE, Drewes AM, Christrup LL,
Kreilgaard M. A review of morphine and morphine-6-glucuronide’s pharmacokinetic-pharmacodynamic relationships in experimental and clinical pain. Eur J Pharm Sci. 2015;74:45−62.

7. Trescot AM, Datta S, Lee M, Hansen H. Opioid pharmacology. Pain Physician. 2008;11(2 Suppl):S133−53.

8. Mercadante S. Opioid titration in cancer pain: A critical review. Eur J Pain. 2007;11:823−30.

9. Hwang CS, Kang EM, Ding Y, et al. Patterns of immediate-release and extended-release opioid analgesic use in the management of chronic pain, 2003-2014. JAMA Netw Open. 2018;1:e180216.

10. Brennan MJ. Update on prescription extended-release opioids and appropriate patient selection. J Multidiscip Healthc. 2013;6:265−80.

11. Zhou J, Wang Y, Jiang G. Oxycodone versus morphine for cancer pain titration: A systematic review and pharmacoeconomic evaluation. PLoS One. 2020;15:e0231763.

12. Hanks GW, Conno F, Cherny N, et al. Expert Working Group of the Research Network of the European Association for Palliative Care. Morphine and alternative opioids in cancer pain: the EAPC recommendations. Br J Cancer. 2001;84:587−93.

13. Shang Y, Filizola M. Opioid receptors: Structural and mechanistic insights into pharmacology and signaling. Eur J Pharmacol. 2015;763:206−13.

14. Valentino RJ, Volkow ND. Untangling the complexity of opioid receptor function. Neuropsychopharmacology. 2018;43:2514−20.

15. James A, Williams J. Basic opioid pharmacology - An update. Br J Pain. 2020;14:115−21.

16. Zhang L, Zhang JT, Hang L, Liu T. Mu opioid receptor heterodimers emerge as novel therapeutic targets: Recent progress and future perspective. Front Pharmacol. 2020;11:1078.

17. Gomes I, Filipovska J, Devi LA. Opioid receptor oligomerization. Detection and functional characterization of interacting receptors. Methods Mol Med. 2003;84:157−83.

18. Gupta A, Mulder J, Gomes I, et al. Increased abundance of opioid receptor heteromers after chronic morphine administration. Sci Signal. 2010;3:ra54.

19. Azzam AAH, McDonald J, Lambert DG. Hot topics in opioid pharmacology: mixed and biased opioids. Br J Anaesth. 2019;122:e136−45.

20. de la Iglesia FA, Pace GW, Robinson GL, Huang NY, Stern W, Richards P. Tolerability and efficacy of two synergistic ratios of oral morphine and oxycodone combinations versus morphine in patients with chronic noncancer pain. J Opioid Manag. 2012;8:89−98.

21. Romero A, Miranda HF, Puig MM. Analysis of the opioid-opioid combinations according to the nociceptive stimulus in mice. Pharmacol Res. 2010;61:511−8.

22. Pasternak GW. Preclinical pharmacology and opioid combinations. Pain Med. 2012;13(Suppl 1):S4−11.

23. Bairaktari A, Al Harbi M, Dimitriou V. Combined use of strong opioids for pain relief in cancer patients - A prospective randomized comparative study. M E J Anesth. 2018;25:31−6.

24. Mercadante S. Opioid combination: rationale and possible clinical applications. Ann Palliat Med. 2013;2:189−96.

25. Marinangeli F, Ciccozzi A, Aloisio L, et al. Improved cancer pain treatment using combined fentanyl-TTS and tramadol. Pain Pract. 2007;7:307−12.

26. Silverman S, Raffa RB, Cataldo MJ, Kwarcinski M, Ripa SR. Use of immediate-release opioids as supplemental analgesia during management of moderate-to-severe chronic pain with buprenorphine transdermal system. J Pain Res. 2017;10:1255−63.

27. de Barutell C, Gonzalez-Escalada J. Efficacy and safety of buprenorphine TDS in conjunction with oral tramadol or morphine as rescue medication in the treatment of 390 patients with chronic pain: A summary of two retrospective Spanish multicenter studies. J Appl Ther Res. 2007;6:14−24.

28. Mercadante S, Villari P, Ferrera P, et al. Safety and effectiveness of intravenous morphine for episodic breakthrough pain in patients receiving transdermal buprenorphine. J Pain Symptom Manage. 2006;32:175−9.