What is Myonal (eperisone hydrochloride)?

What is Myonal (Eperisone Hydrochloride)?

Myonal is the brand name for eperisone hydrochloride, a centrally-acting muscle relaxant used to treat muscle spasm, contracture, and pain associated with musculoskeletal conditions, particularly low back pain. 1, 2

Mechanism of Action

Eperisone works through multiple complementary mechanisms that distinguish it from traditional muscle relaxants:

• Inhibits mono- and multisynaptic reflexes in the spinal cord and supraspinal structures, reducing involuntary muscle contraction 2
• Blocks calcium channels in skeletal muscle, directly reducing muscle tone 3
• Regulates blood supply to skeletal muscles by preventing compression of small blood vessels during contracture, thereby reducing ischemia-induced release of pain mediators 2

This triple mechanism addresses both the neurological and vascular components of muscle spasm, which is particularly important since muscle contracture can compress blood vessels and create a pain-perpetuating cycle of ischemia and inflammation. 2

Clinical Indications

Primary approved uses include:

• Acute low back pain with muscle contracture 2, 4
• Moderate to severe musculoskeletal pain (often combined with paracetamol) 5
• Muscle spasm and spasticity 6
• Chronic low back pain 4

Dosing

Standard adult dosing is 50 mg three times daily (every 8 hours) for acute conditions, typically administered for 10 consecutive days 2. For more severe cases, the dose may be increased to 100 mg three times daily 4.

Key Clinical Advantages

The most clinically significant feature of eperisone is its lack of central nervous system sedation, which is the primary limitation of traditional muscle relaxants like benzodiazepines or cyclobenzaprine 2, 4. This allows patients to maintain normal daily activities without drowsiness or cognitive impairment.

Pharmacokinetics

• Eperisone demonstrates high within-subject variability with CVwR of 33.17% for AUC and 50.21% for Cmax 3
• The drug is extensively metabolized by CYP3A4 and FMO3, with approximately 84% eliminated through hepatic metabolism 7
• Binds to human serum albumin with a binding constant of approximately 10⁴, indicating high affinity, primarily through hydrophobic interactions at Sudlow Site I (subdomain IIA) 6

Clinical Efficacy

In clinical studies of 100 patients with acute low back pain:

• Prompt reduction in both spontaneous and provoked pain within 3 days 2
• Progressive muscle decontracture as evidenced by decreased resistance to passive movement 2
• Reduced antalgic rigidity and improved "hand-to-floor" distance measurements 2
• Improved lower back mobility with decreased rigidity 4

Safety Profile

Eperisone is notably well-tolerated with minimal adverse effects:

• Only 4% discontinuation rate due to minor gastrointestinal reactions in acute low back pain studies 2
• In larger studies, adverse reactions included light-headedness (1%), occasional vertigo/loss of equilibrium (3%), mild somnolence (2%), and epigastric pain (1%) 4
• Most adverse reactions resolve spontaneously without treatment discontinuation 4
• No clinically relevant CNS sedation, distinguishing it from traditional muscle relaxants 2, 4

Clinical Context

Eperisone represents a valuable alternative to traditional analgesics and muscle relaxants because it achieves both analgesic and muscle relaxant effects as a single agent, whereas conventional rheumatology practice typically requires combining a pain killer with a separate muscle relaxant 4. The improved tolerability profile compared to NSAIDs and lack of significant CNS adverse effects make it particularly suitable for patients who need to maintain alertness and function during treatment. 4

Formulations

Eperisone is available in both sugar-coated and film-coated tablet formulations (50 mg), which are bioequivalent and can be used interchangeably 3. The drug is also formulated in combination with paracetamol (eperisone 50 mg + paracetamol 325 mg) for enhanced analgesic effect in moderate to severe pain. 5