Side Effects of Methocarbamol
Methocarbamol causes significant CNS depression (drowsiness, dizziness, sedation) and cardiovascular effects (bradycardia, hypotension) that are particularly dangerous in patients with neurological conditions, seizure history, or brain injury, and should be avoided in patients with hepatic or renal dysfunction due to impaired elimination. 1, 2
Common Central Nervous System Effects
The most frequently reported adverse effects involve CNS depression 2:
- Drowsiness and sedation are the most common side effects 2, 3
- Dizziness or lightheadedness occurs frequently 2
- Confusion and amnesia can develop, particularly concerning in patients with pre-existing neurological conditions 2
- Mild muscular incoordination may impair motor function 2
- Insomnia paradoxically occurs in some patients 2
Serious Neurological Adverse Events
Seizures (including grand mal) have been reported with methocarbamol use, making it particularly hazardous in patients with seizure history or brain injury 2. Additional serious neurological effects include:
Cardiovascular Effects
Methocarbamol produces significant cardiovascular depression that warrants caution in vulnerable populations 1, 2:
- Bradycardia (slow heart rate) 1, 2
- Hypotension (low blood pressure) 1, 2
- Syncope (fainting) 2
- Flushing 2
- Thrombophlebitis with IV administration 2
These cardiovascular effects are why methocarbamol should be held on the day of surgery due to potential interactions with anesthetic agents 1.
Gastrointestinal and Hepatic Effects
- Nausea and vomiting are common 2
- Dyspepsia (indigestion) 2
- Jaundice (including cholestatic jaundice) has been reported, indicating potential hepatotoxicity 2
Hypersensitivity and Immunologic Reactions
Serious allergic reactions can occur 2:
- Anaphylactic reaction 2
- Angioneurotic edema 2
- Hypersensitivity reactions 2
- Urticaria (hives) 2
- Pruritus (itching) 2
- Rash 2
Hematologic Effects
- Leukopenia (low white blood cell count) has been reported 2
Sensory and Other Effects
Critical Drug Interactions and Special Warnings
The combination of methocarbamol with alcohol is contraindicated and potentially fatal due to additive CNS depression 4. A documented case report demonstrated fatal interaction between methocarbamol and ethanol, with combined sedative-hypnotic properties leading to severe CNS depression 4.
Special Population Considerations
Patients with Renal or Hepatic Dysfunction
Methocarbamol elimination is significantly impaired in patients with liver and kidney disease, making it inappropriate for these populations 1, 5. The IV formulation contains polyethylene glycol (PEG), which has been implicated in metabolic acidosis and nephrotoxicity, though the evidence supporting this remains debated 6.
Elderly Patients
All muscle relaxants, including methocarbamol, increase fall risk in elderly patients due to CNS effects 1, 7, 5. However, if a muscle relaxant is necessary in elderly patients, methocarbamol or metaxalone may be safer choices than cyclobenzaprine due to lower anticholinergic burden 7, 5.
Patients with Cardiovascular Disease
Methocarbamol should be used with extreme caution in patients with cardiovascular conditions due to its bradycardia and hypotension effects 1, 2.
Abuse Potential
While methocarbamol is not a controlled substance, it does have some abuse potential at doses well above therapeutic levels (up to 9-12 grams), though this is limited by dysphoric side effects at high doses and is less than that of benzodiazepines like lorazepam 8.
Clinical Pitfalls to Avoid
- Never combine methocarbamol with alcohol due to potentially fatal CNS depression 4
- Hold methocarbamol on the day of surgery to avoid cardiovascular complications and anesthetic interactions 1
- Avoid in patients with hepatic or renal dysfunction due to impaired elimination 1, 5
- Exercise extreme caution in patients with seizure history or brain injury given the risk of seizures 2
- Monitor for fall risk in elderly patients due to sedation and dizziness 1, 7, 5