A generation ago, scientists discovered that the nervous system produces its own morphine-like, pain-relieving substances—the endorphins—which latch onto receptors on the surface of selected nerve cells to initiate their analgesic action. These “opioid” receptors come in several subtypes given Greek letter names (mu, kappa, delta) and respond to a range of opioid drugs such as morphine and codeine. The problem is that cells with opioid receptors are abundant in the nervous system where they serve multiple functions, and prescription opioids are not selective.