Generic Name : Chlorzoxazone.

Trade Name    : Myofen.

Drug Class      : Skeletal Muscle Relaxants.

Forms of The Drug

  1. Tablet: 375mg, 500mg, 750mg.

Mechanism of Action

Chlorzoxazone is a centrally acting muscle relaxant used primarily to relieve discomfort associated with acute musculoskeletal conditions such as muscle spasms, strains, and sprains. Its mechanism of action involves modulation of neurotransmission within the central nervous system (CNS) and inhibition of muscle reflex activity. Here’s how chlorzoxazone works:

  1. GABAergic Activity: Chlorzoxazone enhances the activity of gamma-aminobutyric acid (GABA), the primary inhibitory neurotransmitter in the CNS. It is thought to exert its muscle relaxant effects by facilitating GABAergic neurotransmission, which leads to increased neuronal inhibition and reduced excitability within the spinal cord and supraspinal areas. This inhibition helps to dampen hyperactive reflexes and reduce muscle spasms.
  2. Inhibition of Reflex Arcs: Chlorzoxazone interferes with the transmission of nerve impulses along the spinal cord, particularly within the spinal polysynaptic reflex pathways involved in muscle tone regulation. By inhibiting these reflex arcs, chlorzoxazone suppresses the exaggerated muscle contractions and involuntary movements characteristic of muscle spasms.
  3. No Direct Effect on Skeletal Muscle: Unlike some other muscle relaxants that act directly on skeletal muscle fibers, such as dantrolene, chlorzoxazone does not exert a direct effect on the contractile apparatus of skeletal muscle. Instead, its muscle relaxant effects are primarily mediated through CNS-mediated mechanisms, making it less likely to cause muscle weakness or impair muscle function.
  4. Minimal Sedative Effects: Chlorzoxazone is relatively selective for GABAergic neurotransmission within the CNS, which helps to minimize sedative effects compared to other muscle relaxants that act on GABA receptors more broadly. However, some individuals may still experience mild sedation or drowsiness as a side effect of chlorzoxazone, particularly at higher doses or with concurrent use of other CNS depressants.
  5. Analgesic Effects: In addition to its muscle relaxant properties, chlorzoxazone may possess mild analgesic effects. By reducing muscle spasms and associated pain, chlorzoxazone can help alleviate discomfort in individuals with musculoskeletal conditions.

Indications

  1. Muscle Spasms.
  2. Musculoskeletal Pain: such as strains, sprains, and other injuries to the muscles or bones.
  3. Musculoskeletal Disorders: such as fibromyalgia, myofascial pain syndrome, or tension headaches.
  4. Adjunctive Therapy: such as physical therapy, rest, and analgesic medications.
  5. Postoperative Pain.

Contraindications

  1. Hypersensitivity
  2. Hepatic Impairment.
  3. Renal Impairment.
  4. Porphyria
  5. Children under the age of 12 years.
  6. Pregnancy and Lactation.

Side Effects

  1. Lightheadedness. 
  2. Dizziness. 
  3. Drowsiness. 
  4. Excitement.
  5. Paradoxical. 
  6. Somnolence. 
  7. Malaise. 
  8. Rash. 
  9. Angioneurotic edema. 
  10. Gastrointestinal hemorrhage (rare). 
  11. Hepatotoxicity (rare). 
  12. Anaphylaxis (rare).

Drug Interaction

  1. Central nervous system depressants, such as opioids, benzodiazepines, alcohol, and certain antipsychotic medications.
  2. Cytochrome P450 Inhibitors: such as cimetidine, fluvoxamine, and disulfiram.
  3. Cytochrome P450 enzymes, such as rifampin, phenytoin, and carbamazepine.
  4. Alcohol
  5. Antihypertensive medications: such as beta-blockers, calcium channel blockers, and alpha-blockers.

Precautions

  1. Pregnancy: Category C.
  2. Lactation: There is limited information available regarding the excretion of chlorzoxazone into human breast milk. As a result, caution is advised when considering its use in lactating women.

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