<p>Morphine is 35% protein bound, the metabolite morphine-3-glucuronide is 10% protein bound, and morphine-6-glucuronide is 15% protein bound.<sup class="text-reference-group"><a class="reference-popover-link" data-content="Klimas R, Mikus G: Morphine-6-glucuronide is responsible for the analgesic effect after morphine administration: a quantitative review of morphine, morphine-6-glucuronide, and morphine-3-glucuronide. Br J Anaesth. 2014 Dec;113(6):935-44. doi: 10.1093/bja/aeu186. Epub 2014 Jul 1. (PubMed ID 24985077)" href="#reference-A176059">4</a></sup></p>