Do endogenous opioids mediate or fine-tune human pain relief?

We discuss how effects of different magnitudes from human opioid antagonist studies can be interpreted and how the results inform us on the role of the endogenous mu-opioid system for pain relief and other affective processes in humans. This is a commentary on Endogenous opioids contribute to the feeling of pain relief in humans by Sirucek et al, In Press, Pain.