What is the rate of side effects with methocarbamol (muscle relaxant)?

Rate of Side Effects with Methocarbamol

Methocarbamol has a moderate side effect profile with common adverse reactions including drowsiness (most frequent), dizziness, lightheadedness, and nausea, affecting approximately 30-40% of patients. 1

Common Side Effects

Methocarbamol's adverse reactions can be categorized by body system:

• Nervous system (most common):

  • Drowsiness
  • Dizziness/lightheadedness
  • Mild muscular incoordination
  • Confusion
  • Insomnia
  • Vertigo
  • Seizures (rare)

• Cardiovascular system:

  • Hypotension
  • Bradycardia
  • Syncope
  • Flushing
  • Thrombophlebitis

• Digestive system:

  • Nausea and vomiting
  • Dyspepsia
  • Jaundice (including cholestatic jaundice)

• Other systems:

  • Blurred vision
  • Headache
  • Rash
  • Pruritus
  • Urticaria
  • Metallic taste
  • Nasal congestion 1

Incidence Rates

In clinical studies, side effects of methocarbamol have been reported at the following rates:

• In a randomized double-blind trial comparing methocarbamol to diazepam for acute low back pain, drowsiness was reported in only 4% of methocarbamol patients compared to 29.4% in the diazepam group 2

• A 1975 double-blind trial found that side effects with methocarbamol (1500 mg four times daily) occurred at almost equal incidence to placebo 3

• In a study evaluating methocarbamol for acute low back pain, no severe adverse effects were observed (only 7 symptoms in 5 patients) 4

Special Populations and Risk Factors

Certain populations have increased risk of adverse effects:

• Patients with liver or kidney disease: Methocarbamol elimination is significantly impaired, increasing risk of drug accumulation and adverse effects 5

• Patients with cardiovascular disease: Use with caution due to documented cardiovascular effects including bradycardia and hypotension 5

• Patients taking CNS depressants: Methocarbamol's sedative properties can enhance CNS depression when combined with other sedatives 5

• Alcohol interaction: A potentially fatal interaction between methocarbamol and alcohol has been documented, with combined central nervous system depression due to interactive sedative-hypnotic properties 6

Abuse Potential

Methocarbamol has some potential for abuse, though less than benzodiazepines:

• At doses well above therapeutic levels, methocarbamol can produce statistically significant dose-related increases in subjects' ratings of drug effect and liking
• However, this abuse potential is likely decreased by accompanying side effects at high doses
• Methocarbamol's abuse potential appears to be less than that of lorazepam 7

Clinical Implications

When prescribing methocarbamol:

1. Start with the lowest effective dose to minimize side effects
2. Warn patients about potential drowsiness and advise against driving or operating machinery
3. Avoid combining with alcohol or other CNS depressants when possible
4. Consider holding methocarbamol on the day of surgical procedures due to cardiovascular effects
5. Use with caution in patients with liver or kidney disease, considering dose reduction
6. Monitor for signs of drug accumulation in susceptible populations

The side effect profile of methocarbamol is generally favorable compared to other muscle relaxants, particularly benzodiazepines, with drowsiness being less frequent but still the most common adverse effect.