Table 1.

Opioid Receptors: Agonists, Antagonists, Side Effects.

Receptor	Endogenous Peptides	Opioid Agonists	Effects	Side Effects
µ mu [MOR]/Oprm1	β-Endorphins Endomorphin-1 Endomorphin-2	Morphine Fentanyl Meperdine Methadone Codeine Oxymorphone Hydromorphone Levorphanol	Analgesia	Tolerance and dependence after chronic exposure to opioid agonists Respiratory depression Nausea Urinary retention Biliary spasms and constipation Sedation Reward/euphoria
K kappa [KOR]/Oprk1	Dynorphin-A Dynorphin-B	Morphine Levorphanol	Analgesia	Tolerance and dependence after chronic exposure to opioid agonists Diuretic effect Sedative effect Dysphoria
δ delta [DOR]/Oprd1	Leu-enkephalin Met-enkephalin	Meperidine Codeine Hydromorphone Levorphanol	Analgesia Spinal analgesia	Tolerance and dependence after chronic exposure to opioid agonists Antidepressant effect Reward/Euphoria Respiratory depression Constipation
NOP	N/OFQ	—	Analgesia (spinally)	Exacerbation of chronic pain condition Tolerance to morphine Depression Hyperalgesia (low dose of N/OFQ) Parkinson’s disease