13.2 Substance Use Disorders: Opioid Agonist Therapy, Harm Reduction & Alcohol Withdrawal

Substance Use Disorders: Opioid Agonist Therapy, Harm Reduction & Alcohol Withdrawal

Canada faces an ongoing public health crisis driven by an increasingly toxic, unpredictable unregulated drug supply dominated by illicit synthetic fentanyl, novel synthetic benzodiazepines, and high-potency stimulants. Pharmacists stand at the critical intersection of clinical pharmacotherapy and harm reduction. Competence in Opioid Agonist Therapy (OAT), acute withdrawal management, harm reduction modalities, and relapse prevention for Alcohol Use Disorder (AUD) is essential for Canadian pharmacy practice, guided by national consensus standards from the Canadian Research Initiative in Substance Misuse (CRISM) and provincial regulatory authorities.

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Opioid Use Disorder (OUD): Canadian Treatment Hierarchy

CRISM national guidelines establish a clear hierarchical framework for the pharmacological management of Opioid Use Disorder in adults and adolescents:

                       CRISM OUD PHARMACOTHERAPY HIERARCHY

     FIRST-LINE THERAPY               SECOND-LINE THERAPY              THIRD-LINE THERAPY
┌───────────────────────────┐    ┌───────────────────────────┐    ┌───────────────────────────┐
│   BUPRENORPHINE /         │    │         METHADONE         │    │    SLOW-RELEASE ORAL      │
│   NALOXONE (Suboxone)     │───►│       (Full Agonist)      │───►│      MORPHINE (SROM)      │
│ • Partial mu-agonist      │    │ • Unsurpassed retention   │    │ • Kadian 24-hr capsules   │
│ • Respiratory ceiling     │    │ • No ceiling effect       │    │ • Witnessed applesauce or │
│ • Take-home safety        │    │ • QTc & CYP3A4 monitoring │    │   swallowed whole         │
└───────────────────────────┘    └───────────────────────────┘    └───────────────────────────┘

1. First-Line Therapy: Buprenorphine / Naloxone (Suboxone)

• Pharmacology:
  • Buprenorphine: A high-affinity partial agonist at the mu-opioid receptor and an antagonist at the kappa-opioid receptor. Because of its partial intrinsic activity, buprenorphine activates the mu receptor sufficiently to relieve opioid cravings and suppress withdrawal, while displaying a ceiling effect on respiratory depression.
  • Naloxone (4:1 ratio): Added strictly as an abuse deterrent. When taken sublingually as prescribed, naloxone exhibits negligible oral/sublingual bioavailability (< 10%) due to extensive first-pass hepatic glucuronidation, exerting zero clinical effect. However, if the tablet or film is crushed and injected intravenously or snorted, naloxone exhibits high bioavailability and precipitates severe, immediate opioid withdrawal in opioid-dependent individuals.
• Clinical Advantages Over Methadone:
  • Substantially lower risk of fatal overdose during treatment induction and maintenance;
  • Favorable cardiovascular safety (minimal QTc prolongation);
  • Accelerated eligibility for take-home doses ("carries"), fostering patient autonomy and treatment retention;
  • Mild adverse effect profile and fewer clinically significant cytochrome P450 drug interactions.

Pathophysiology of Precipitated Withdrawal

Precipitated withdrawal occurs when an antagonist or high-affinity partial agonist is introduced into a patient whose mu-opioid receptors are currently occupied by a full agonist (e.g., fentanyl, heroin, oxycodone, methadone).

                    MECHANISM OF PRECIPITATED WITHDRAWAL

    FULL AGONIST AT RECEPTOR           BUPRENORPHINE DISPLACEMENT            PRECIPITATED WITHDRAWAL
┌──────────────────────────────┐    ┌──────────────────────────────┐    ┌──────────────────────────────┐
│ Full Agonist (e.g., Fentanyl)│    │ Buprenorphine has HIGHER     │    │ Net mu-receptor activation   │
│ occupies mu-receptors with   │───►│ receptor affinity, displaces │───►│ drops instantly from 100%    │
│ 100% intrinsic signal.       │    │ full agonist from receptor.  │    │ to partial agonist level (~40%)│
└──────────────────────────────┘    └──────────────────────────────┘    └──────────────────────────────┘

Because buprenorphine binds with exceptionally high affinity ($K_i \approx 0.2\text{ nM}$), it rapidly knocks the full agonist off the mu receptor. However, because its intrinsic efficacy is partial, the net cellular signaling cascades plummet instantly, precipitating a sudden, violent, excruciating withdrawal state that peaks within 30 to 60 minutes.

Induction Modalities for Buprenorphine/Naloxone

Standard Induction (Traditional)

• Requires the patient to cease all full-agonist opioids and wait until they enter objective moderate opioid withdrawal before taking the first dose of buprenorphine/naloxone.
• Objective Assessment: Monitored using the Clinical Opiate Withdrawal Scale (COWS), requiring a score of ≥ 12 (or Subjective Opiate Withdrawal Scale [SOWS] ≥ 17).
• Timing Guidelines: Typically requires a minimum of 12 to 24 hours after short-acting opioids (heroin, immediate-release oxycodone/hydromorphone), and 48 to 72+ hours after long-acting opioids (methadone).
• Day 1 Dosing: Administer an initial test dose of 2 mg to 4 mg sublingually. Observe the patient for 60 to 90 minutes. If withdrawal improves and no precipitated withdrawal occurs, titrate in 2 to 4 mg increments up to a total Day 1 dose of 8 mg to 12 mg. Rapidly titrate on subsequent days to a target maintenance dose of 12 mg to 24 mg daily (Health Canada maximum approved dose is 24 mg daily; provincial guidelines occasionally permit up to 32 mg daily under specialist care).

Low-Dose Micro-Induction (The "Bernese Method")

• In the era of unregulated illicit fentanyl, standard induction has become exceedingly difficult because fentanyl is highly lipophilic and accumulates extensively in adipose tissue, leaching into the circulation for days to weeks. Even patients with COWS > 12 frequently experience severe precipitated withdrawal during standard induction.
• Core Principle: Overlapping, escalating micro-doses of buprenorphine are administered while the patient continues their full-agonist opioid at regular doses without cessation. Buprenorphine gradually accumulates and binds to an increasing percentage of mu receptors at sub-clinical levels without displacing enough full agonist to trigger withdrawal. Once a therapeutic buprenorphine dose (e.g., 12 to 16 mg daily) is reached, the full agonist is abruptly discontinued.

                   REPRESENTATIVE BERNESE MICRO-INDUCTION PROTOCOL

   Day 1:  0.5 mg SL once daily   + Full-agonist opioid continued as usual
   Day 2:  0.5 mg SL twice daily  + Full-agonist opioid continued as usual
   Day 3:  1.0 mg SL twice daily  + Full-agonist opioid continued as usual
   Day 4:  2.0 mg SL twice daily  + Full-agonist opioid continued as usual
   Day 5:  4.0 mg SL twice daily  + Full-agonist opioid continued as usual
   Day 6:  8.0 mg SL twice daily  + Full-agonist opioid continued as usual
   Day 7:  12 mg to 16 mg SL once daily + DISCONTINUE full-agonist opioid completely
   Day 8+: Titrate to maintenance (16 mg to 24 mg daily)

Extended-Release Subcutaneous Buprenorphine (Sublocade)

• Following successful stabilization on sublingual buprenorphine (at least 8 mg to 24 mg daily for a minimum of 7 consecutive days), patients can transition to monthly extended-release subcutaneous depot buprenorphine (Sublocade).
• Dosing Regimen: Typically initiated with two monthly loading doses of 300 mg SC, followed by a monthly maintenance dose of 100 mg SC (can be maintained at 300 mg monthly if clinically indicated).
• Clinical Benefit: Delivers stable, continuous plasma concentrations over 28 to 35 days, eliminating daily dosing peaks and troughs, safeguarding against diversion, and completely decoupling recovery from daily pharmacy visits.

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Second-Line Therapy: Methadone Maintenance Therapy (MMT)

Methadone is a synthetic full mu-opioid receptor agonist that also functions as an NMDA receptor antagonist and an inhibitor of serotonin and norepinephrine reuptake.

Pharmacokinetics & Safety Parameters

• Half-Life: Highly lipophilic with extensive extravascular tissue distribution; elimination half-life is long and exhibits extreme inter-individual variability, ranging from 15 to 60+ hours (average 24 to 36 hours).
• Steady-State Kinetics: Requires 5 to 7 days (or up to 2 weeks) of consistent daily dosing to achieve pharmacokinetic steady state after any initiation or dose titration.
• Lack of Ceiling Effect: As a full agonist, methadone has no ceiling effect on respiratory depression. Dose escalation during early treatment carries a significant risk of fatal accumulation and overdose.
• Dosing Rule ("Start Low, Go Slow"): Initial doses should be 10 mg to 30 mg daily (rarely exceeding 30 mg, even in heavy users, due to variable baseline tolerance). Doses should be adjusted by no more than 5 mg to 10 mg every 5 to 7 days.

Critical Clinical Toxicities & Drug Interactions

1. QTc Interval Prolongation & Torsades de Pointes:
   • Methadone causes dose-dependent blockade of the human ether-à-go-go-related gene (hERG) potassium channels, prolonging ventricular repolarization (QTc interval).
   • Clinicians should obtain a baseline 12-lead ECG, particularly when daily doses exceed 100 mg to 120 mg daily, in patients with baseline cardiac disease, or when combined with other QTc-prolonging drugs (e.g., ciprofloxacin, fluconazole, macrolides, citalopram, ondansetron, antipsychotics).
2. Cytochrome P450 Drug Interactions:
   • Methadone is cleared primarily by hepatic CYP3A4, with secondary contributions from CYP2B6, CYP2C19, and CYP2D6.
   • Enzyme Inducers (Accelerate Clearance & Precipitate Withdrawal): Rifampin, carbamazepine, phenytoin, phenobarbital, efavirenz, and St. John's wort dramatically reduce methadone plasma concentrations, causing acute opioid withdrawal within 24 to 72 hours.
   • Enzyme Inhibitors (Impair Clearance & Cause Overdose): Clarithromycin, ketoconazole, itraconazole, fluconazole, ciprofloxacin, and fluvoxamine impair methadone clearance, precipitating life-threatening opioid toxicity without any change in prescribed dose.

Pharmacy Dispensing, Take-Home Doses & Missed Doses Protocol

• Formulation: Dispensed as an oral liquid (commonly 10 mg/mL concentrate diluted to 100 mL in a vehicle such as orange crystals / Tang to obscure taste and deter intravenous injection).
• Witnessed Ingestion vs. Carries: Initiated exclusively under daily witnessed ingestion at the community pharmacy dispensary. Patients earn take-home doses ("carries") progressively upon demonstrating clinical stability, treatment engagement, negative urine drug screens, stable housing, and possession of a locked storage box.

                  CANADIAN MISSED METHADONE DOSES PROTOCOL

    1 TO 2 DAYS MISSED                3 OR 4 DAYS MISSED                 ≥ 5 DAYS MISSED
┌───────────────────────────┐    ┌───────────────────────────┐    ┌───────────────────────────┐
│ • Tolerance is maintained │    │ • Tolerance drops sharply │    │ • Tolerance fully lost    │
│ • Dispense FULL normal    │───►│ • DOSE MUST BE REDUCED    │───►│ • DO NOT DISPENSE         │
│   prescribed dose         │    │   (typically reduced by   │    │ • Prescriber reassessment │
│ • Resume daily schedule   │    │   50% or re-titrated)     │    │   restart at 10–30 mg     │
└───────────────────────────┘    └───────────────────────────┘    └───────────────────────────┘

3. Third-Line Therapy: Slow-Release Oral Morphine (SROM - Kadian)

• Indicated for patients with severe OUD who have failed or have contraindications/intolerance to both buprenorphine/naloxone and methadone.
• Administration: Prescribed as 24-hour pellets inside capsules (Kadian). Dispensed under strict daily witnessed ingestion. The capsule is opened in the presence of the pharmacist, and the intact pellets are sprinkled onto a spoonful of applesauce or swallowed directly with water. Pellets must never be chewed, crushed, dissolved, or heated, as destroying the pellet membrane results in immediate dose dumping and fatal overdose.

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Harm Reduction Frameworks & Community Interventions

Harm reduction encompasses evidence-based public health interventions aimed at reducing the negative medical, social, and economic consequences associated with substance use, without demanding immediate abstinence.

1. Take-Home Naloxone (THN) Distribution

Take-Home Naloxone kits are universally accessible without a prescription across Canadian community pharmacies through publicly funded programs (e.g., Ontario Naloxone Program, BC Take Home Naloxone Program).

• Kit Types:
  • Intramuscular (IM) Kit: Contains two to three 0.4 mg/mL ampoules or vials of naloxone hydrochloride, safety syringes with retractable needles (VanishPoint), alcohol swabs, and a rescue breathing barrier mask.
  • Intranasal (IN) Spray Kit: Contains two single-dose nasal spray devices delivering 4 mg of naloxone hydrochloride per 0.1 mL actuation.
• Crucial Pharmacokinetic Directive: Naloxone exhibits an elimination half-life of only 30 to 90 minutes. In contrast, synthetic fentanyl analogues and methadone exert central respiratory depressant effects for 6 to 24+ hours. As naloxone dissociates from the mu receptor, the patient will relapse into respiratory arrest (re-narcotization). Calling emergency medical services (911) is non-negotiable.

2. Sterile Consumption Supplies & Secondary Prevention

Community pharmacies actively distribute sterile drug-using equipment:

• Sterile needles and syringes (mitigating transmission of HIV, Hepatitis B, and Hepatitis C);
• Cookers, sterifilt filters, and sterile water ampoules (preventing bacterial endocarditis, sepsis, and localized abscesses);
• Safer inhalation supplies (heat-resistant pyrex stems, silicone mouthpieces, wooden push sticks, mitigating lip lacerations, burns, and blood-borne virus transmission);
• Drug checking strips (fentanyl test strips, benzodiazepine test strips) to identify adulterants in the illicit drug supply.

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Alcohol Use Disorder (AUD): Acute Withdrawal Management

Alcohol (ethanol) enhances central inhibitory GABA-A receptor transmission and inhibits excitatory NMDA glutamate receptors. Chronic alcohol exposure causes compensatory down-regulation of GABA-A receptors and up-regulation of NMDA receptors. Abrupt cessation unmasks an acute neurochemical imbalance characterized by profound GABA deficiency and excessive glutamate neurotransmission, precipitating central nervous system hyperexcitability and autonomic hyperactivity.

                  CHRONOLOGY OF ALCOHOL WITHDRAWAL SYMPTOMS

   6–12 Hours              12–24 Hours             24–48 Hours              48–96 Hours
┌───────────────┐       ┌───────────────┐       ┌───────────────┐       ┌───────────────┐
│ Tremors       │       │ Alcoholic     │       │ Withdrawal    │       │ DELIRIUM      │
│ Sweating      │──────►│ Hallucinosis  │──────►│ Seizures      │──────►│ TREMENS (DTs) │
│ Tachycardia   │       │ (Auditory/    │       │ (Generalized  │       │ Severe tremor,│
│ Nausea        │       │  Visual; clear│       │  tonic-clonic)│       │ fever, altered│
│ Anxiety       │       │  sensorium)   │       │               │       │ sensorium     │
└───────────────┘       └───────────────┘       └───────────────┘       └───────────────┘

Clinical Institute Withdrawal Assessment for Alcohol, Revised (CIWA-Ar)

• A validated 10-item instrument measuring withdrawal severity (nausea/vomiting, tremor, paroxysmal sweats, anxiety, agitation, tactile disturbances, auditory disturbances, visual disturbances, headache, orientation).
• Scoring Tiers:
  • Mild Withdrawal: Score < 10 (generally managed with supportive care, quiet environment, oral hydration);
  • Moderate Withdrawal: Score 10 to 18 (warrants pharmacological intervention);
  • Severe Withdrawal: Score ≥ 19 (high risk for withdrawal seizures and delirium tremens; warrants aggressive inpatient pharmacotherapy).

Pharmacotherapy of Acute Withdrawal: Benzodiazepines

• Regimen Approach: Symptom-triggered therapy (doses administered strictly when CIWA-Ar ≥ 10) is strongly preferred over fixed-schedule regimens, resulting in shorter treatment duration, lower total benzodiazepine consumption, and reduced sedation.
• Agent Selection:
  • Diazepam (Oral or IV): Standard agent of choice in patients with preserved liver function. It has a rapid onset of action and a very long elimination half-life with active metabolites (nordiazepam half-life up to 100 hours), providing an "auto-tapering" effect that protects against breakthrough rebound withdrawal.
  • Lorazepam (Oral, SL, or IV): Agent of choice in patients with advanced liver disease, cirrhosis, severe hepatic encephalopathy, elderly patients, or respiratory failure. Unlike diazepam and chlordiazepoxide (which require hepatic Phase I microsomal oxidation), lorazepam undergoes direct Phase II glucuronidation without active metabolites, eliminating the danger of profound hepatic drug accumulation and prolonged coma.

Prevention of Wernicke-Korsakoff Encephalopathy: Thiamine Sequencing

• Pathophysiology: Chronic heavy alcohol use leads to severe thiamine (Vitamin B1) deficiency secondary to poor nutritional intake, impaired intestinal absorption, and reduced hepatic storage. Thiamine pyrophosphate is an obligatory cofactor for key enzymes in cerebral glucose metabolism (transketolase, pyruvate dehydrogenase).
• Clinical Presentation: Wernicke Encephalopathy presents with the classic triad of ophthalmoplegia/nystagmus, ataxia, and acute confusion/encephalopathy. If untreated, it progresses to irreversible Korsakoff Syndrome (profound anterograde and retrograde amnesia with confabulation).

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Alcohol Relapse Prevention Pharmacotherapy

Following successful completion of acute withdrawal, evidence-based pharmacotherapies are indicated to support long-term abstinence or reduction of heavy drinking days:

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Clinical Case Scenario: Navigating Missed Methadone Doses & OAT Transition

A 34-year-old male registered in a community pharmacy methadone maintenance program presents on Tuesday morning requesting his daily dose of methadone 85 mg. The pharmacy dispensing record reveals his last witnessed ingestion was on Friday morning (he missed Saturday, Sunday, and Monday—3 consecutive missed days). He admits he relapsed on illicit fentanyl over the weekend. He is anxious, exhibits mild piloerection and dilated pupils, but is alert and oriented.

Pharmacist Clinical Assessment & Action Plan:

1. Assessment of Tolerance: Because the patient has missed 3 consecutive daily doses, his metabolic and pharmacodynamic tolerance to methadone has dropped substantially. Administering his full 85 mg dose carries a severe risk of lethal respiratory arrest.
2. Immediate Dispensing Action: The pharmacist cannot dispense the full 85 mg dose. Under provincial guidelines (e.g., OCP, BCCSU), the pharmacist contacts the prescribing physician. Standard protocol dictates reducing the dose by approximately 50% (e.g., 40 mg to 45 mg) for the first day, followed by rapid step-up titration (e.g., 60 mg Day 2, 75 mg Day 3, 85 mg Day 4) provided doses are witnessed and tolerated.
3. Urine Drug Screening: Perform a clinical point-of-care or laboratory urine drug screen to assess concurrent substance exposure (fentanyl, benzodiazepines).
4. Harm Reduction Intervention: Review overdose risks, ensure the patient possesses an active, unexpired take-home naloxone kit, and discuss whether transitioning to buprenorphine/naloxone (via low-dose Bernese micro-induction) or long-acting injectable buprenorphine (Sublocade) would offer superior safety and flexibility.