Substance Use Disorders

Neurobiology of Addiction

Addiction is a chronic, relapsing brain disease characterized by compulsive drug seeking and use despite harmful consequences. The neurobiological framework centers on the brain’s reward, motivation, and executive control circuits.

The Reward Pathway (Mesolimbic Dopamine System)

The ventral tegmental area (VTA), nucleus accumbens (NAc), and prefrontal cortex (PFC) form the core circuitry of addiction. All addictive substances, directly or indirectly, increase dopamine transmission in the nucleus accumbens.

Brain Region	Function in Addiction	Effect of Chronic Use
Ventral Tegmental Area (VTA)	Dopamine neuron origin; triggered by drugs to release dopamine in NAc	Reduced baseline dopamine firing; tolerance develops
Nucleus Accumbens (NAc)	Reward processing and salience attribution	Downregulation of D2 receptors; anhedonia during withdrawal
Prefrontal Cortex (PFC)	Executive control, impulse inhibition, decision-making	Hypofrontality; impaired self-regulation and increased impulsivity
Amygdala	Stress response, negative affect	Heightened stress reactivity during withdrawal
Hippocampus	Memory formation of drug-related cues	Strong cue-conditioned memories that trigger craving

The Addiction Cycle

Stage	Neurobiological Substrate	Behavioral Manifestation
Binge/Intoxication	VTA-NAc dopamine activation, opioid peptide release	Euphoria, reward seeking
Withdrawal/Negative Affect	Amygdala CRF increase, reduced dopamine function	Dysphoria, anxiety, irritability
Preoccupation/Anticipation	PFC glutamatergic projections to NAc	Craving, compulsive drug seeking

DSM-5 Diagnostic Criteria for Substance Use Disorder

The DSM-5 integrates abuse and dependence into a single disorder on a severity continuum. Criteria are grouped under four categories: impaired control, social impairment, risky use, and pharmacological criteria (tolerance and withdrawal).

The 11 Criteria

Domain	Criterion	Description
Impaired Control	1	Substance taken in larger amounts or over longer periods than intended
Impaired Control	2	Persistent desire or unsuccessful efforts to cut down or control use
Impaired Control	3	Great deal of time spent obtaining, using, or recovering from substance
Impaired Control	4	Craving or strong desire to use the substance
Social Impairment	5	Failure to fulfill major role obligations at work, school, or home
Social Impairment	6	Continued use despite persistent social or interpersonal problems
Social Impairment	7	Important social, occupational, or recreational activities given up
Risky Use	8	Recurrent use in hazardous situations
Risky Use	9	Continued use despite knowledge of physical or psychological problem
Pharmacological	10	Tolerance (need for increased dose or diminished effect)
Pharmacological	11	Withdrawal (characteristic syndrome or use to relieve/avoid withdrawal)

Severity Classification

Severity	Number of Criteria Met
Mild	2–3 criteria
Moderate	4–5 criteria
Severe	6 or more criteria

Specifiers include “in early remission” (3–12 months) and “in sustained remission” (12+ months), as well as “on maintenance therapy” and “in a controlled environment.”

Alcohol Use Disorder

Alcohol is a central nervous system depressant that primarily acts through potentiation of GABA-A receptors and inhibition of NMDA glutamate receptors.

Mechanism of Action

Neurotransmitter System	Acute Effect of Alcohol	Chronic Adaptation	Withdrawal Effect
GABA (inhibitory)	Enhanced GABA-A receptor activity (increased Cl- influx)	Downregulation of GABA-A receptors	Reduced inhibition; anxiety, seizures
Glutamate (excitatory)	NMDA receptor antagonism	Upregulation of NMDA receptors	Glutamate storm; neurotoxicity, excitotoxicity
Dopamine	Increased release in NAc	D2 receptor downregulation	Anhedonia, dysphoria
Serotonin	Altered 5-HT3 receptor activity	Receptor adaptation	Mood instability

Alcohol Withdrawal Syndrome

Withdrawal occurs 6–24 hours after last drink and can be life-threatening.

Stage	Timeframe	Symptoms	Management
Mild (Stage 1)	6–12 hours	Tremor, anxiety, nausea, insomnia, palpitations	Supportive care, thiamine, monitoring
Moderate (Stage 2)	12–24 hours	Hallucinations (visual, tactile, auditory), diaphoresis, hypertension	Benzodiazepines (CIWA-Ar protocol)
Severe (Stage 3)	24–72 hours	Delirium tremens (DTs): confusion, severe autonomic hyperactivity, seizures, hyperthermia	ICU-level care, high-dose benzodiazepines, phenobarbital

Pharmacotherapy for Alcohol Use Disorder

Medication	Mechanism	Efficacy	Contraindications
Naltrexone	Mu-opioid receptor antagonist; reduces craving and drinking	Reduces heavy drinking days (NNT ~12)	Opioid use, acute hepatitis
Acamprosate	NMDA receptor modulator; reduces withdrawal-related distress	Maintains abstinence (NNT ~11)	Severe renal impairment
Disulfiram	Aldehyde dehydrogenase inhibitor; causes aversive reaction	Effective with supervised dosing, adherence is poor	Alcohol intoxication, coronary disease
Gabapentin	GABA analog; reduces craving and sleep disturbances	Moderate benefit for heavy drinkers	Renal impairment

Opioid Use Disorder

Opioids include prescription pain relievers (oxycodone, hydrocodone, morphine), heroin, and synthetic opioids (fentanyl). They act primarily at mu-opioid receptors (MOR).

Neurobiology of Opioids

Receptor Type	Primary Effect	Clinical Significance
Mu (MOR)	Analgesia, euphoria, respiratory depression, miosis, constipation	Primary target for opioids; responsible for addictive potential
Kappa (KOR)	Dysphoria, sedation, psychotomimetic effects	May contribute to stress-induced relapse
Delta (DOR)	Analgesia, antidepressant-like effects	Less implicated in addiction

Opioid Overdose

Respiratory depression is the primary cause of death in opioid overdose. The triad of respiratory depression, miosis (pinpoint pupils), and depressed consciousness is classic.

Overdose Sign	Description
Respiratory depression	< 8 breaths per minute, shallow breathing, agonal gasping
Miosis	Pinpoint pupils (may be absent with hypoxia or mixed ingestions)
Depressed consciousness	Unresponsiveness, stupor, coma
Cyanosis	Blue lips or fingertips due to hypoxia
Bradycardia	Slow heart rate

Naloxone (Narcan) is a competitive mu-opioid receptor antagonist. It reverses respiratory depression within 2–5 minutes of administration.

Naloxone Route	Dose	Onset	Duration	Notes
Intranasal	4 mg (2 mg per nostril)	2–5 min	30–90 min	Preferred for lay administration
Intramuscular	0.4–2 mg	2–5 min	30–90 min	Standard for EMS
Intravenous	0.04–0.4 mg	1–2 min	20–60 min	Titrate to avoid precipitated withdrawal

Duration of naloxone (30–90 minutes) may be shorter than the opioid’s duration, requiring repeat dosing and immediate transport to an emergency department.

Medication-Assisted Treatment (MAT)

Medication	Mechanism	Dosing	Efficacy	Considerations
Methadone	Full mu-opioid agonist; reduces craving and withdrawal	Daily dosing in OTP; 60–120 mg/day typical	Reduces illicit opioid use, mortality, and HIV transmission	QTc prolongation; requires daily clinic visits
Buprenorphine	Partial mu-opioid agonist; ceiling effect on respiratory depression	Sublingual tablet/film; 8–24 mg/day	Reduces overdose mortality by ~50%	Precipitated withdrawal if started too early
Naltrexone (XR)	Mu-opioid antagonist; blocks opioid effects	Monthly IM injection (380 mg)	Effective for highly motivated patients; no diversion potential	Requires complete detoxification before initiation

Cannabis Use Disorder

Cannabis acts through cannabinoid receptors (CB1 and CB2). Delta-9-tetrahydrocannabinol (THC) is the primary psychoactive component.

Endocannabinoid System

Component	Function	Effect of Cannabis
CB1 receptor	Primarily CNS: modulates neurotransmitter release (GABA, glutamate, dopamine)	THC binding produces euphoria, altered perception, impaired memory
CB2 receptor	Peripheral and immune cells: modulates inflammation	Less psychoactive; potential therapeutic target
Anandamide (AEA)	Endogenous CB1 agonist; “bliss molecule”	THC mimics anandamide but with higher potency and longer duration
2-AG	Endogenous CB1 agonist; abundant in CNS	THC disrupts normal endocannabinoid signaling

Cannabis Withdrawal Syndrome

Symptom	Prevalence	Time Course
Irritability, anger, aggression	> 80%	Onset 24–72 hours; peaks day 2–6; resolves in 1–2 weeks
Anxiety, nervousness	> 70%	Same time course
Sleep difficulty (insomnia, vivid dreams)	> 75%	May persist 2–4 weeks
Decreased appetite, weight loss	> 70%	Resolves in 1–2 weeks
Restlessness	> 65%	Peaks first week
Depressed mood	> 50%	Variable; may require monitoring

Stimulant Use Disorder: Cocaine vs. Methamphetamine

Property	Cocaine	Methamphetamine
Mechanism	Blocks dopamine, norepinephrine, serotonin reuptake via DAT, NET, SERT	Enters presynaptic neuron via DAT/VMAT2; releases dopamine and prevents breakdown
Onset of action	Rapid (seconds if smoked/IV; minutes if intranasal)	Rapid; prolonged effects
Duration of high	15–30 minutes (smoked/IV); 60–90 minutes (intranasal)	8–24 hours
Half-life	~1 hour	~10–12 hours
Metabolism	Plasma esterases, hepatic	Hepatic (CYP2D6)
Neurotoxicity	Moderate; primarily cardiovascular risk	High: oxidative stress, mitochondrial damage, dopamine terminal degeneration
Withdrawal	Dysphoria, fatigue, hypersomnia, craving (1–7 days)	Severe anhedonia, psychomotor retardation, craving (1–4 weeks)
Medical complications	MI, stroke, arrhythmia, seizure	Severe dental disease (“meth mouth”), skin picking, cardiovascular damage, stroke

Benzodiazepine Use Disorder

Benzodiazepines potentiate GABA-A receptor activity, producing anxiolytic, sedative, and anticonvulsant effects.

BZD Class	Examples	Onset	Half-Life	Abuse Potential
Short-acting	Triazolam, midazolam	Rapid (15–30 min)	2–6 hours	High
Intermediate	Alprazolam, lorazepam, temazepam	Intermediate (30–60 min)	6–20 hours	High
Long-acting	Diazepam, chlordiazepoxide, clonazepam	Slow to intermediate	20–100+ hours	Moderate

Benzodiazepine Withdrawal

Withdrawal can be severe and life-threatening, including seizures and delirium. Tapering is essential.

Withdrawal Symptom	Management
Rebound anxiety, insomnia	Slow taper over weeks to months
Diaphoresis, tachycardia	Vital sign monitoring
Seizures	Anticonvulsant medications; prevent with slow taper
Delirium, psychosis	ICU-level care, benzodiazepine reinstatement

Treatment Levels of Care (ASAM Criteria)

The American Society of Addiction Medicine (ASAM) defines a continuum of care.

Level	Description	Setting	Typical Duration
0.5	Early intervention	Outpatient	Single sessions
1	Outpatient treatment (OP)	Clinic or office	1–3 hours/week; 3–6 months
2.1	Intensive outpatient (IOP)	Structured program	9–20 hours/week; 2–6 months
2.5	Partial hospitalization (PHP)	Day program	20+ hours/week; 2–4 months
3.1	Clinically managed low-intensity residential	Group home, halfway house	3–12 months
3.3	Clinically managed population-specific	Specialized programs	Variable
3.5	Clinically managed high-intensity residential	Residential treatment	1–6 months
3.7	Medically monitored intensive inpatient	Withdrawal management (detox)	3–14 days
4	Medically managed intensive inpatient	Hospital	3–30 days

Behavioral Therapies for Substance Use Disorders

Cognitive-Behavioral Therapy (CBT)

CBT for SUD targets dysfunctional beliefs about substances and develops coping skills.

Component	Description
Functional analysis	Identify antecedents (triggers) and consequences of substance use
Skills training	Drug refusal skills, coping with craving, problem-solving
Cognitive restructuring	Challenge beliefs that justify use (“one won’t hurt”)
Relapse prevention	Identify high-risk situations; develop alternative responses
Lifestyle modification	Exercise, sleep hygiene, recreational activities

Motivational Interviewing (MI)

MI is a client-centered, directive approach that strengthens intrinsic motivation for change.

MI Principle	Description	Technique
Express empathy	Understand the client’s perspective without judgment	Reflective listening, affirmations
Develop discrepancy	Highlight gap between current behavior and personal values/gods	Elicit pros and cons of use vs. change
Roll with resistance	Avoid arguing for change; invite new perspectives	Reframing, shifting focus
Support self-efficacy	Instill belief that change is possible	Emphasize past successes, strengths

Contingency Management (CM)

CM provides tangible incentives for objective evidence of behavior change (e.g., negative urine drug screens).

Element	Description
Target behavior	Abstinence (confirmed by drug testing), treatment attendance, medication adherence
Incentives	Vouchers, cash, prizes, clinic privileges
Schedule	Escalating reinforcement (increasing value with consecutive successes)
Effectiveness	Strongest evidence among all behavioral interventions for stimulant use disorders

Evidence Summary by Substance

Substance	First-Line Psychotherapy	Pharmacotherapy
Alcohol	CBT, MI, 12-step facilitation	Naltrexone, acamprosate, disulfiram
Opioids	CM (for abstinence initiation), CBT	Buprenorphine, methadone, XR-naltrexone
Cannabis	CBT, MI, CM (incentives for negative UDS)	None FDA-approved
Cocaine	CM (strongest evidence), CBT	None FDA-approved
Methamphetamine	CM (strongest evidence), CBT	None FDA-approved
Benzodiazepines	CBT, relapse prevention	Slow taper (e.g., diazepam taper)