Is there an intravenous (IV) version of Flexeril (cyclobenzaprine)?

No IV Formulation of Cyclobenzaprine (Flexeril) Exists

There is no intravenous (IV) formulation of cyclobenzaprine (Flexeril) available for clinical use. Cyclobenzaprine is only available in oral formulations, including tablets, extended-release capsules, and oral suspensions.

Available Formulations of Cyclobenzaprine

• Oral tablets: 5 mg, 7.5 mg, and 10 mg
• Extended-release capsules: 15 mg, 30 mg
• No injectable or IV formulation exists

Alternative Muscle Relaxants with IV Formulations

If an IV muscle relaxant is required, methocarbamol is the only centrally-acting skeletal muscle relaxant with an FDA-approved IV formulation 1:

• Methocarbamol (Robaxin): Available in both oral and IV formulations
  • IV dosing: 1-3 g initially, followed by 1 g every 8 hours as needed
  • Primary side effects: drowsiness, dizziness, and cardiovascular effects (bradycardia, hypotension)
  • Generally has fewer anticholinergic effects than cyclobenzaprine

Clinical Considerations When Selecting Muscle Relaxants

Efficacy Comparison

• Cyclobenzaprine is effective for short-term relief of acute muscle spasm, with onset of relief apparent within 3-4 doses of the 5 mg regimen 2
• Tizanidine is considered the most effective alternative to cyclobenzaprine for treating low back pain and muscle spasms 1

Side Effect Profile

• Cyclobenzaprine's primary side effects include anticholinergic effects (hallucinations, confusion), dry mouth, constipation, urinary retention, and sedation 1
• Sedation is the most common adverse event with cyclobenzaprine, occurring in over 30% of patients 3
• Recent research has shown that cyclobenzaprine is a potent non-competitive antagonist of histamine H1 receptors, which likely explains its significant sedative effects 3

Contraindications

Cyclobenzaprine is contraindicated in patients with:

• MAO inhibitor use (current or within 14 days of discontinuation)
• Acute recovery phase of myocardial infarction
• Heart block, conduction disturbances, or arrhythmias
• Hyperthyroidism 1

Clinical Algorithm for Muscle Relaxant Selection When IV Administration Is Required

1. First choice: Methocarbamol IV if immediate parenteral administration is needed
2. Second choice: Consider oral cyclobenzaprine if the patient can take oral medications
3. Special situations:
   • For patients with liver concerns: Avoid tizanidine and chlorzoxazone; consider methocarbamol with monitoring
   • For patients with sedation concerns: Methocarbamol may have less sedative effect than cyclobenzaprine
   • For patients with spasticity rather than simple muscle spasm: Consider baclofen

Important Cautions

• Serotonin syndrome has been reported with cyclobenzaprine, particularly when combined with other serotonergic medications like SSRIs 4
• All skeletal muscle relaxants are associated with CNS adverse effects, primarily sedation 1
• Short-term use (≤21 days) of muscle relaxants is recommended whenever possible 1

Remember that cyclobenzaprine's mechanism of action is primarily central, affecting tonic somatic motor function through modulation of noradrenergic and serotonergic systems 3, making it effective for acute musculoskeletal pain but also contributing to its side effect profile.