Substance use disorders (SUDs) involve a cluster of cognitive, behavioral, and physiological symptoms indicating that the individual continues using a substance despite significant substance-related problems. Addiction is a chronic, relapsing brain disorder characterized by compulsive substance seeking and use despite harmful consequences.

Stress and anxiety — Substance use disorders are chronic, relapsing brain disorders characterized by compulsive drug seeking and use. Source: Unsplash.

Neurobiology of Addiction

The reward pathway — the mesolimbic dopamine system originating in the ventral tegmental area (VTA) and projecting to the nucleus accumbens (NAc) and prefrontal cortex — is central to the reinforcing effects of all addictive substances. All drugs of abuse increase dopamine in the NAc, producing euphoria and reinforcing drug-seeking behavior. With repeated use, the brain adapts: dopamine receptor availability decreases, leading to tolerance (needing more to achieve the same effect) and anhedonia (reduced ability to experience pleasure from natural rewards). The prefrontal cortex, which mediates executive control and decision-making, undergoes hypofunction, reducing the ability to inhibit drug-seeking impulses. The extended amygdala, involved in stress and negative affect, becomes hyperactive during withdrawal, promoting negative reinforcement — use to relieve distress. This three-stage cycle — binge/intoxication, withdrawal/negative affect, and preoccupation/craving — underlies addiction as a chronic relapsing condition.

DSM-5 Diagnostic Criteria

Substance use disorder is diagnosed using 11 criteria organized into four groups. Impaired control: taking more or for longer than intended, persistent desire or unsuccessful efforts to cut down, spending a great deal of time obtaining or using, and craving. Social impairment: failure to fulfill major role obligations, continued use despite interpersonal problems, and giving up important activities. Risky use: recurrent use in hazardous situations and continued use despite physical or psychological problems. Pharmacological criteria: tolerance and withdrawal (not counted for prescribed medications taken as directed).

Severity. Mild: 2–3 criteria. Moderate: 4–5 criteria. Severe: 6 or more criteria.

Alcohol Use Disorder

Alcohol is the most commonly used addictive substance. AUD affects 5–10% of the population and is a leading cause of preventable death worldwide.

Intoxication. Disinhibition, impaired coordination, slurred speech, ataxia, nystagmus, stupor, and coma at high levels. Blood alcohol concentration (BAC) of 0.08% (legal limit for driving) causes significant impairment. Emergency care required for severe intoxication with respiratory depression or coma.

Withdrawal. Onset 6–12 hours after last drink, peaking at 24–72 hours. Symptoms: autonomic hyperactivity (tachycardia, hypertension, sweating, fever), tremor, insomnia, nausea, anxiety, agitation. Withdrawal seizures occur in 3–5% of untreated patients. Alcohol withdrawal delirium (delirium tremens): confusion, severe agitation, hallucinations, autonomic instability, and risk of death. The Clinical Institute Withdrawal Assessment for Alcohol (CIWA-Ar) guides benzodiazepine treatment. Thiamine supplementation prevents Wernicke encephalopathy and Korsakoff syndrome.

Treatment. Withdrawal management (benzodiazepines, thiamine, folic acid, multivitamins). Psychosocial interventions: cognitive behavioral therapy, motivational enhancement therapy, 12-step facilitation (Alcoholics Anonymous). Pharmacotherapy: naltrexone (reduces craving and heavy drinking), acamprosate (reduces withdrawal-related distress), and disulfiram (aversive agent causing acetaldehyde accumulation if alcohol is consumed).

Opioid Use Disorder

The opioid epidemic has caused hundreds of thousands of overdose deaths. Fentanyl and its analogs are driving current fatalities.

Intoxication and overdose. Opioid overdose causes respiratory depression (the lethal effect), pinpoint pupils, and CNS depression. Naloxone (Narcan) is a mu-opioid receptor antagonist that rapidly reverses overdose and is available as a nasal spray for community use.

Withdrawal. Onset 8–24 hours after last use, peaking at 36–72 hours. Symptoms: dysphoria, anxiety, muscle aches, lacrimation, rhinorrhea, yawning, sweating, diarrhea, nausea, dilated pupils, and piloerection. Withdrawal is extremely uncomfortable but not life-threatening (unlike alcohol withdrawal). The Clinical Opiate Withdrawal Scale (COWS) guides treatment.

Treatment. Medication-assisted treatment (MAT) is the gold standard. Methadone is a full mu-opioid agonist provided through regulated clinics. Buprenorphine is a partial mu-opioid agonist with a ceiling effect — safer in overdose but can still cause respiratory depression when combined with benzodiazepines or alcohol. Naltrexone extended-release injection blocks opioid effects and is used after detoxification. Psychosocial support and counseling enhance outcomes. Overdose education and naloxone distribution save lives.

⚠ Clinical Correlation

Harm reduction is a set of practical strategies that reduce the negative consequences of drug use without requiring abstinence. Key interventions: needle and syringe exchange programs (reduce HIV and hepatitis C transmission by 50–80%), supervised consumption sites (prevent overdose deaths, connect users to treatment), naloxone distribution (community availability prevents overdose fatalities), fentanyl test strips (allow users to check for fentanyl in supplies), and low-threshold buprenorphine (same-day access without requiring counseling engagement). Harm reduction is evidence-based, cost-effective, and does not increase substance use. It is endorsed by the WHO, CDC, and major medical organizations.

Cannabis Use Disorder

Cannabis is the most commonly used illicit substance. CUD develops in 9% of all cannabis users, rising to 17% of those who begin in adolescence and 25–50% of daily users.

Effects. Acute: euphoria, relaxation, impaired short-term memory, impaired coordination, increased appetite, and anxiety or paranoia in some. Chronic: amotivational syndrome, respiratory effects from smoking, cognitive impairment that may partially reverse with abstinence (especially in those who begin use in adolescence), and cannabis hyperemesis syndrome (cyclical nausea and vomiting relieved by hot showers).

Treatment. Psychosocial treatments — CBT, motivational enhancement therapy, and contingency management — are the mainstay. No FDA-approved pharmacotherapy exists for CUD.

Stimulant Use Disorders

Cocaine and methamphetamine cause intense euphoria through massive dopamine release. Chronic use leads to dopamine depletion and severe anhedonia. Physical effects: tachycardia, hypertension, coronary vasospasm, myocardial infarction, stroke, and seizures. Long-term methamphetamine use causes prominent cognitive deficits and psychiatric symptoms (paranoia, hallucinations, compulsive behaviors). No FDA-approved pharmacotherapy exists for stimulant use disorders. Contingency management — which provides tangible incentives for drug-free urine samples — is the most effective behavioral intervention.

Benzodiazepine Use Disorder

Benzodiazepines are prescribed for anxiety and insomnia but have addiction potential, especially with long-term use. Withdrawal is similar to alcohol withdrawal — seizures and delirium tremens can occur. Management requires slow, supervised taper (often over months), switching to a long-acting benzodiazepine, and monitoring for anxiety and insomnia.

★ Key Concept

Substance use disorders commonly co-occur with other mental health conditions (dual diagnosis). Depression, anxiety, PTSD, bipolar disorder, and personality disorders all have high rates of comorbidity with SUDs. Bidirectional relationships exist — mental illness increases SUD risk, and substance use can precipitate or worsen mental illness. Integrated treatment, in which both conditions are treated by the same team in the same setting, is more effective than sequential or parallel treatment. Principles: treat both conditions as primary; use evidence-based pharmacotherapy for both; coordinate care; and use motivational approaches to engage ambivalent patients.

Levels of Care

The American Society of Addiction Medicine (ASAM) defines levels of care: early intervention, outpatient, intensive outpatient, partial hospitalization, residential/inpatient, and medically managed intensive inpatient. The appropriate level depends on acute intoxication/withdrawal potential, biomedical conditions, emotional/behavioral conditions, readiness to change, relapse potential, and recovery environment.

Summary

Substance use disorders are chronic brain diseases involving dysregulation of reward, stress, and executive control systems. The DSM-5 provides a dimensional approach with 11 criteria and three severity levels. Alcohol and opioids cause the most morbidity and mortality. Medication-assisted treatment is the gold standard for opioid use disorder. Harm reduction approaches reduce death and disease transmission. Integrated treatment for co-occurring disorders improves outcomes. Recovery is possible with comprehensive, evidence-based care.