opioid use disorder

OUD involves neuroadaptive changes in the mesolimbic dopamine reward pathway, particularly the ventral tegmental area and nucleus accumbens, driven by chronic mu-opioid receptor agonism. Repeated use leads to downregulation of receptors, tolerance, and dependence, with withdrawal mediated largely by locus coeruleus noradrenergic hyperactivity when opioids are discontinued (causing symptoms like lacrimation, rhinorrhea, piloerection, diarrhea, mydriasis, and autonomic instability—classic exam findings). DSM-5 criteria include impaired control (using more than intended, unsuccessful attempts to cut down), social impairment (failure to fulfill obligations), risky use, and pharmacological criteria (tolerance, withdrawal)—note tolerance/withdrawal alone in patients on prescribed pain medication under medical supervision does not count toward diagnosis. Severity is classified as mild (2-3 symptoms), moderate (4-5), or severe (6+). Complications include overdose (respiratory depression, miosis, CNS depression—classic triad), infectious complications from IV use (endocarditis, HIV, hepatitis C), neonatal abstinence syndrome in pregnant patients, and constipation. First-line treatment is medication-assisted treatment: buprenorphine (partial mu agonist, ceiling effect reduces overdose risk, often combined with naloxone as Suboxone), methadone (full agonist, requires specialized clinic dosing), and naltrexone (mu antagonist, requires full detoxification first). Naloxone is used for acute overdose reversal. Behavioral therapy and harm reduction (needle exchange programs) are adjuncts. High-yield board associations: understanding the difference between physical dependence/tolerance and addiction, recognizing withdrawal timeline (short-acting opioids: 6-12 hr onset, peak 1-3 days; methadone: delayed onset up to 36 hrs), and management of neonatal abstinence syndrome with morphine or methadone tapering.