1. 1) United Nations Office on Drugs and Crime. “World drug report 2019; 3 depressants.”: ‹https://wdr.unodc.org/wdr2019/prelaunch/WDR19_Booklet_3_DEPRESSANTS.pdf›, accessed 24 September, 2020.
2. 2) United Nations Office on Drugs and Crime. “Fentanyl and its analogues—50 years on. Global Smart Update.”: ‹http://www.unodc.org/documents/scientific/Global_SMART_Update_17_web.pdf›, accessed 24 September, 2020.
3. 3) Zhang R, Xie X. Tools for GPCR drug discovery. Acta Pharmacol. Sin., 33, 372–384 (2012).
4. 4) Siegfried. “Synthesis of fentanyl.”: ‹https://erowid.org/archive/rhodium/chemistry/fentanyl.html›, accessed 24 September, 2020.
5. 5) Golla R, Seethala R. A homogeneous enzyme fragment complementation cyclic AMP screen for GPCR agonists. J. Biomol. Screen., 7, 515–525 (2002).