^Dhawan BN, Cesselin F, Raghubir R, Reisine T, Bradley PB, Portoghese PS, Hamon M. International Union of Pharmacology. XII. Classification of opioid receptors. Pharmacol. Rev. December 1996, 48 (4): 567–92. PMID 8981566.
^Janecka A, Fichna J, Janecki T. Opioid receptors and their ligands. Curr Top Med Chem. 2004, 4 (1): 1–17. PMID 14754373.