Where it acts: the spinal cord

Placed into the fluid around the spinal cord it behaves as a potent mu agonist at the level where pain signals are handed from the body to the brain. That location is why the documented human use is spinal: a very small amount, placed exactly where the receptors are, instead of a large amount circulating everywhere. Pain relief measured in people lasted about 43 hours from a single placement, against about 34 hours for intrathecal morphine and 11 hours for an injected painkiller into muscle. Precision of placement, not quantity, is what produced that.