What muscle relaxant can be safely used with methadone (opioid analgesic)?

Muscle Relaxants to Use with Methadone

When selecting a muscle relaxant for patients on methadone, methocarbamol is the safest option due to its minimal drug interactions and low risk of respiratory depression or QTc prolongation.

Safety Considerations with Methadone

• Methadone has unique pharmacological properties including a long half-life (8-120 hours), high potency, and significant interindividual variations in pharmacokinetics 1
• High doses of methadone (≥120 mg) may lead to QTc prolongation and torsades de pointes, potentially causing sudden cardiac death 1
• Methadone is metabolized by CYP3A4 and CYP1A2 enzymes, making drug interactions a significant concern 2
• Patients on methadone should avoid medications that:
  • Prolong QTc interval 1
  • Inhibit CYP3A4 metabolism of methadone 1
  • Have significant CNS depressant effects that could compound respiratory depression 1

Muscle Relaxant Options

Methocarbamol (Recommended)

• Methocarbamol is the preferred muscle relaxant for patients on methadone due to its:
  • Minimal effect on QTc interval 1
  • No significant CYP3A4 interactions 1
  • Lower risk of respiratory depression compared to other muscle relaxants 1
• Dosing considerations:
  • Hold on the day of surgical procedures 1
  • Use with caution in patients with liver or kidney disease as elimination may be impaired 1
  • Should not be used in patients with myasthenia gravis 1

Metaxalone (Alternative Option)

• Can be considered as an alternative when methocarbamol is contraindicated 1
• Centrally acting with unclear mechanism of action 1
• Contraindicated in patients with significant hepatic or renal dysfunction 1
• Hold on day of surgery if applicable 1

Muscle Relaxants to Avoid with Methadone

Cyclobenzaprine

• Should be avoided due to:
  • Structural similarity to tricyclic antidepressants (TCAs) 1
  • Potential for QTc prolongation when combined with methadone 1
  • Anticholinergic effects that may compound side effects 1
  • Risk of serotonin syndrome with other serotonergic medications 1

Carisoprodol

• Avoid due to:
  • Risk of respiratory depression that could compound with methadone 1
  • Abuse and addiction potential 1
  • Withdrawal symptoms with prolonged use 1
  • CNS depressant effects that could increase sedation when combined with methadone 1

Orphenadrine

• Avoid due to:
  • Anticholinergic properties that may worsen side effects 1
  • Potential for cardiovascular instability that could compound with methadone's cardiac effects 1
  • Risk of QTc prolongation when combined with methadone 1

Monitoring Recommendations

• Baseline and follow-up electrocardiogram for patients on methadone when:
  • Adding any new medication, including muscle relaxants 1
  • Patient has cardiac disease 1
  • Patient is taking other medications known to prolong QTc 1
• Monitor for signs of respiratory depression, especially during the first 4-7 days after adding a muscle relaxant 1
• Consider alternative opioids if QTc is >500 msec or if QTc is 450-500 msec with other risk factors 1

Special Considerations

• For patients with acute pain requiring additional analgesia beyond methadone:
  • Continue the patient's daily methadone maintenance dose 1
  • Add short-acting opioid analgesics as needed 1
  • Avoid mixed agonist-antagonist opioids (pentazocine, nalbuphine, butorphanol) as they may precipitate withdrawal 1
• If muscle spasm is severe and requires stronger intervention, consider non-pharmacological approaches (physical therapy, heat/cold therapy) before adding medications with higher risk profiles 1