The opioid receptor family comprises μ (MOR), δ (DOR), κ (KOR), and nociceptin/orphanin FQ receptor (NOP), all belonging to the G protein-coupled receptor class. They play a crucial role in regulating pain, emotions, reward sensations, and dependence. These receptors are widely distributed in tissues such as the brain, spinal cord, and digestive system, and they alleviate pain by inhibiting neuronal activity. Activation of opioid receptors influences various signaling pathways, including cAMP, potassium ion channels, and calcium ion channels. Gene expression is influenced by genetics and physiological states, and mutations or expression variations are associated with pain sensitivity, differences in drug effects, and addiction tendencies. Drug development targeting opioid receptors aims to treat pain while minimizing the risk of addiction.