4.3 Medications for Opioid Use Disorder (MOUD: Methadone, Buprenorphine & Extended-Release Naltrexone)

Medications for Opioid Use Disorder (MOUD: Methadone, Buprenorphine & Extended-Release Naltrexone)

Overview & Evidence Base for MOUD

Pharmacotherapy for Opioid Use Disorder—historically termed Medication-Assisted Treatment (MAT) and now preferentially designated Medications for Opioid Use Disorder (MOUD)—is the gold standard of evidence-based care. Extensive clinical trials demonstrate that long-term MOUD retention:

• Reduces all-cause mortality by 50% to 70%
• Reduces fatal opioid overdose risk by over 60%
• Decreases bloodborne pathogen transmission (HIV, Hepatitis C) through reduced injection drug use
• Improves treatment retention, criminal justice outcomes, and sustained employment

The three FDA-approved pharmacotherapies for OUD represent three distinct pharmacological classes: Methadone (full mu-agonist), Buprenorphine (partial mu-agonist), and Naltrexone (full mu-antagonist).

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Methadone: Full Agonist & OTP Regulatory Framework

Pharmacology & Dosing

Methadone is a synthetic, long-acting full mu-opioid receptor agonist and NMDA receptor antagonist. It eliminates opioid craving and withdrawal symptoms without producing euphoria in tolerant individuals due to its slow absorption and extended elimination half-life (24 to 36 hours).

• Induction Dosing: Typically started at 10–30 mg daily. Dose titrations occur slowly (every 3–5 days in increments of 5–10 mg) to prevent cumulative overdose toxicity as tissue equilibrium is established.
• Therapeutic Maintenance Range: Typically 60 to 120 mg daily. Doses below 60 mg are generally ineffective for suppression of craving and blockade of illicit opioid effects.

Federal Regulatory Framework (42 CFR Part 8)

Under federal law, methadone for OUD can ONLY be dispensed through SAMHSA-certified, DEA-registered Opioid Treatment Programs (OTPs). Patients must attend the clinic daily for supervised oral dosing. Over time, patients earn "take-home" doses based on SAMHSA stability criteria (abstinence from illicit substances, program compliance, stable housing, personal safety).

Inpatient Hospitalization Exception: Under the "3-day rule" (DEA 21 CFR 1306.07), a non-OTP acute care hospital nurse may administer methadone to a hospitalized patient to maintain OUD stability while treating a primary medical condition (e.g., endocarditis, abscess).

Cardiac Safety & QTc Monitoring

Methadone inhibits the human ether-à-go-go-related gene (hERG) cardiac potassium channel, causing dose-dependent QTc interval prolongation and increasing the risk of Torsades de Pointes (a fatal ventricular dysrhythmia).

• Nursing ECG Protocol: Obtain baseline 12-lead ECG prior to initiation. Re-assess ECG at doses >100 mg/day, or if adding QTc-prolonging medications (e.g., macrolides, fluoroquinolones, haloperidol, SSRIs).
• Action Thresholds: If QTc is 450–500 ms, discuss risk/benefit and monitor closely. If QTc exceeds 500 ms, reduce methadone dose or transition to buprenorphine.

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Buprenorphine: Partial Agonist & The 2023 MAT Act Shift

Pharmacology & Ceiling Effect

Buprenorphine is a partial mu-opioid agonist and kappa-opioid antagonist. Because it is a partial agonist, buprenorphine exhibits a receptor activation ceiling effect: beyond a certain dose (typically 16–24 mg sublingually), further dose increases yield no additional respiratory depression or euphoria, rendering it exceptionally safe against fatal overdose.

The MAT Act Regulatory Elimination (2023)

Enacted in December 2022 and implemented in 2023, the Mainstreaming Addiction Treatment (MAT) Act eliminated the federal requirement for prescribers to obtain a special DATA 2000 "X-waiver" to prescribe buprenorphine. All DEA-registered clinicians with Schedule III prescribing authority can now prescribe buprenorphine for OUD within standard practice, dramatically expanding patient access.

Sublingual Combination Formulations

To prevent intravenous misuse, buprenorphine is co-formulated with naloxone in a 4:1 ratio (e.g., Suboxone: 8 mg buprenorphine / 2 mg naloxone).

• Sublingual Route: Naloxone has minimal sublingual bioavailability (<3%), allowing buprenorphine to exert full therapeutic effects.
• Parenteral Misuse (IV Injection): If dissolved and injected intravenously, naloxone becomes 100% bioavailable, blocking mu-receptors and triggering immediate severe withdrawal in opioid-dependent individuals.

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Extended-Release Naltrexone: Full Antagonist Protocols

Pharmacology & Administration

Naltrexone is a competitive full mu-opioid receptor antagonist that completely blocks the euphoric and physiological effects of exogenous opioids.

• Extended-Release Injectable (Vivitrol): Administered as a 380 mg deep intramuscular (IM) gluteal injection every 4 weeks (28 days). ER-naltrexone eliminates daily adherence challenges associated with oral naltrexone (50 mg/day).
• Opioid-Free Requirement: Patients MUST BE COMPLETELY OPIOID-FREE for 7 to 14 days prior to initiating naltrexone (7 days for short-acting opioids; 10–14 days for methadone or buprenorphine). Administering naltrexone prematurely triggers catastrophic precipitated withdrawal.

Safety Warning - Overdose Risk Upon Relapse: Naltrexone therapy causes up-regulation of mu-opioid receptor density. If a patient discontinues Vivitrol and relapses to opioids, their loss of tolerance makes them extremely vulnerable to fatal overdose from opioid doses they previously tolerated.

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FDA-Approved MOUD Comparison Matrix