(1) Collins Concise English Dictionary. Copyright by HarperCollins Publishers
(2) WordReference Random House Unabridged Dictionary of American English. Copyright 2016.
(3) WordReference Random House Learner’s Dictionary of American English. Copyright 2016.
(4) Wikipedia, the free encyclopedia. All definitions taken from this site may be found under the same name as the one defined. Retrieved between July 1st and 3rd., 2016.
(5) Holden, JE, Jeong Y., Forrest, JM. (2005). The endogenous opioid system and clinical pain management. AACN Clin Issues. 2005 Jul-Sep, 16(3): 291. At: http://www.ncbi.nlm.nih.gov/pubmed/16082232 Retrieved: July 1st, 2016.