Endogenous opioids are a class of molecules that are produced in the brain and circulate widely throughout all organ systems. Endogenous opioids are neuropeptides and are derived from one of the two precursor genes—pre-proenkephalin A or pro-opiomelanocortin (POMC).

Subsequently, a plethora of other endogenous opioid peptides were identified. The endogenous opioid system is vital in regulating various physiologic functions like pain relief (analgesia), euphoria induction, stress resilience, cardiovascular protection, food intake control, etc. Three genetically distinct opioid peptides families are considered classical members of the endogenous opioid system:
• Endorphins
• Enkephalins
• Dynorphin
• Kappa