11523
Morphine pharmacology in beta - arrestin - 2 knockout mice suggested that a ligand that promotes coupling of the mu - opioid receptor ( MOR ) to G proteins , but not beta - arrestins , would result in higher analgesic efficacy , less gastrointestinal dysfunction , and less respiratory suppression than morphine .
 Morphine, morphine
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: Chemical 
Text: {0} 
Answer:
 Morphine, morphine