I was reading about the morphine molecule. It binds to our opioid receptors. These opioid receptors have endogenous binding partners also, like β-Endorphin. There have been reports that these endogenous receptors have a different impact on opioid receptors than exogenous opioids (morphine). Source
I couldn't find any trials of intravenously administered β-Endorphins for pain relief. Although there are reports of using intravenous β-Endorphins on mice (1976), monkey (1989) and schizophrenia patient (1980).
Am I missing something? Is it possible to use endogenous opioids?