In vertebrates, endogenous opioids are neuromodulators that moderate pain by interacting with opioid receptors. Buprenorphine works as a partial opioid agonist. Buprenorphine affects different types of opioid receptors in different ways. Codeine is an opioid and an agonist of the mu opioid receptor. Buprenorphine is used to treat people with opioid use disorder. Many patients are unable to access morphine, methadone or an equivalent opioid. Fentanyl is a synthetic opioid structurally similar arylpiperidines. Methadone is used for the treatment of opioid use disorder. Dynorphin may play a role in the inhibition of dopamine release via kappa opioid receptors. Ketorolac is also an adjuvant to opioid medications and improves pain relief. Oxycodone is the most widely recreationally used opioid in America. There are four major subtypes of opioid receptors. It may work on pain via effects on the opioid receptor. This points to a role for opioid receptors in mating behaviors. Exactly how causes the release of endogenous opioid peptides remains uncertain.