It seems opioids work centrally and peripherally. From WP: Opioids act by binding to opioid receptors, which are found principally in the central and peripher

It seems opioids work centrally and peripherally. From WP: Opioids act by binding to opioid receptors, which are found principally in the central and peripheral nervous system and the gastrointestinal tract. Recent investigations discovered opioid receptors on peripheral sensory neurons.A significant fraction (up to 60%) of opioid analgesia can be mediated by such peripheral opioid receptors, particularly in inflammatory conditions such as arthritis, traumatic or surgical pain. Inflammatory pain is also blunted by endogenous opioid peptides activating peripheral opioid receptors. Here are some of the studies mentioned: * https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4565585/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4565585/ * https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4173362/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4173362/ * http://www.nature.com/nm/journal/v9/n8/full/nm908.html http://www.nature.com/nm/journal/v9/n8/full/nm908.html PS: I have no idea of what I'm talking about, just doing some basic research, so take it all with a lot of salt.