Morphine and hydrocodone act as endogenous opioids at much higher potency than kratom, crossing the blood-brain barrier to bind to receptors such as the mu-opioid and kappa opioid receptors.

“Kratom and morphine, and hydrocodone all have the property of acting like the endogenous opioids but at much higher potency. When you bring them into your system, they cross the blood-brain barrier. So they go into the brain, and they bind to a number of different opioid receptors. There's the mu-opioid receptor. By the way, these names all follow Greek alphabet letters. So the mu-opioid receptor, the kappa opioid receptor, there are a bunch of different ones.”