Pain is critical to various clinical pathologies, provoking an urgent need for effective management. Pain, whether acute or chronic, is a complex neurochemical process. Its alleviation depends on the type, with nonopioid analgesics effective for mild to moderate pain, such as musculoskeletal or inflammatory pain, while neuropathic pain responds best to anticonvulsants, tricyclic antidepressants, or serotonin/norepinephrine reuptake inhibitors. For severe acute or chronic pain, opioids may be selected carefully. Despite their potential for misuse due to euphoric properties, opioids remain vital in intense pain relief. Traditionally, opioids were pivotal in acute pain alleviation, but concerns over misuse and addiction have prompted the exploration of alternatives such as NSAIDs, anticonvulsants, and antidepressants. Opiates, products from the poppy plant (Papaver somniferum), and endogenous opioids, naturally occurring ligands for opioid receptors like β-endorphin, are crucial to understanding analgesia. Opioid receptor activation, both acute and chronic, can lead to desensitization, tolerance, dependence, and addiction. Tolerance development varies among physiological responses, with some showing reduced response (analgesia, sedation) and others remaining unaffected (pupil miosis). Receptor disposition, intracellular signaling adaptations, and system-level counteradaptation influence chronic tolerance and dependence. Opioid receptors are G protein-coupled proteins that influence ion channels, Ca²⁺ disposition, and protein phosphorylation. They primarily act at the μ-opioid receptor, though drugs like morphine also interact with δ and κ receptors. To mitigate side effects, receptor-selective agonists, biased agonists, and combinations targeting peripheral opioid receptors have been developed. Furthermore, heterodimerization between μ opioid (MOR) and nonopioid receptors (e.g., NOPs) can produce significant analgesia with reduced adverse effects.

Opioids function at multiple sites to increase analgesic efficacy - inhibiting pain transmission neurons directly, activating descending inhibitory neurons, and modulating pain signals. Peripheral μ receptors on sensory neurons contribute to this effect. Tolerance and physical dependence develop with frequent therapeutic doses of opioids, with the persistent activation of μ receptors playing a pivotal role in its induction and maintenance. Understanding the development of opioid tolerance and dependence remains a challenge.