Methocarbamol is a centrally acting muscle relaxant. It is commonly used in the treatment of patients with acute painful conditions arising from muscle spasms. It is available as oral tablets of 500 mg and 750 mg strength and as an injection for Intramuscular and intravenous use.
It has an onset of action of about 30 minutes when taken orally. It has a half-life of 1-2 hours. The peak plasma concentration is reached in 1 – 2 hours. It is primarily excreted through the kidneys as inactive metabolites.
Methocarbamol MOA (Mechanism of action of Methocarbamol):
Methocarbamol is a centrally acting muscle relaxant. It does not have any effect directly on the striated skeletal muscle fibers. The exact mechanism of action is not known, however, it is a carbamate and has a similar mechanism of action.
It acts on the neuromuscular junction, central nervous system, and autonomic nervous system and inhibits the enzyme acetylcholinesterase. Thus, it modulates the activity of the muscle at the level of the brain and the neuromuscular junction.
It is a centrally-acting muscle relaxant
Methocarbamol (Robaxin) is used to treat a variety of conditions that primarily involve the muscles. It relaxes the muscles but does not have a direct effect on the pain pathways. It is specifically used to treat acute muscle spasms such as in patients with:
- Acute low back pain resulting from a sudden stretch or lifting of a heavy object, inappropriate body posture at night (as in distant travelers sleeping in the sitting position or individuals with a recent change in pillow or mattress).
- Acute muscle spasms arising from trauma or muscle injury.
- Other conditions such as sprains and tetanus.
It is not helpful in patients with chronic conditions of the muscles such as individuals with chronic arthritis resulting in contractures of the joints, cerebral palsy, and degenerative arthritis like osteoarthritis. It is also not helpful in other painful conditions that are not caused by muscle spasms such as toothache.
Muscle relaxants effective in acute low back pain include:
- Carisoprodol, and
Muscle relaxants effective in chest pain caused by a muscular problem:
Methocarbamol in combination with pain medicines:
Methocarbamol is a muscle relaxant. It is usually combined with pain medicine for better efficacy. It is available in combination with:
- Aspirin, and
It is important to note that Methocarbamol is centrally acting and has sedative properties. When combined with other drugs that cause sedation, it might result in drowsiness and impairment of physical and mental abilities.
Methocarbamol Use with Sedative analgesics and Alcohol:
The combined use of narcotic analgesics such as codeine, tramadol, and opioids should be avoided because of the central nervous system depressant effects (except in terminally ill cancer patients). The combination of Methocarbamol and sedative analgesics like opioids may result in serious respiratory depression.
Methocarbamol should also be avoided with alcohol as both drugs have a synergistic effect and may cause serious respiratory depression.
Patients with chronic lung diseases such as those with COPD and obstructive sleep apnea should avoid combining it with sedative analgesics due to the risk of serious respiratory depression.
Methocarbamol may cause serious respiratory depression
Methocarbamol use in Gout:
Methocarbamol has no direct effect on gout. It does not directly reduce or worsen the inflammation of gouty arthritis. Neither does it have any effect on the uric acid levels.
However, the Methocarbamol/ Aspirin combination pill should be avoided in patients with Gout. Aspirin in low doses (75 to 300 mg) increases the reabsorption of the filtered uric acid from the kidneys and inhibits the excretion of uric acid from the kidneys.
Thus, in patients with gout, it is important to check the formulation of methocarbamol and avoid using it if it contains aspirin.
Avoid Robaxin/Aspirin in patients with GOUT
Is Methocarbamol (Robaxin) addictive?
Since methocarbamol is a centrally acting drug and causes sedation, it has the potential to be abused.
In one study, Methocarbamol was compared with Lorazepam (Ativan). It was observed that both drugs caused psychomotor and behavioural changes in the study participants in a dose-dependent manner. When used in supra-therapeutic doses, methocarbamol use was associated with an impairment of cognitive impairment.
Although the central effects were more pronounced in patients in the lorazepam group, it was also seen with methocarbamol. The drug may be addictive if used in high doses, although, the incidence of side effects may limit its use in high doses [Ref].
Robaxin may be addictive at high doses
Methocarbamol dose in the symptomatic treatment of muscle spasm:
It is available as 500 mg and 750 mg tablets. The usual recommended dose is 1500 mg orally three or four times a day for the relief of muscle spasm. In severe condition, up to 8000 mg may be given per day (equivalent to 16 tablets of 500 mg).
The IM and IV injection is given in an initial dose of 1000 mg IM or IV followed by twice or thrice administration. The maximum dose is 3000 mg per day. The IV and IM formulations may be used in children with tetanus at a dose of 15 mg/kg or 500 mg/m²/dose up to a maximum of 1800 mg/m²/day.
Usual Dose is 1500 mg 3 – 4 times a day
Who should not take Methocarbamol (Robaxin):
Methocarbamol should be avoided if the person is allergic to it. It should also be avoided in patients with kidney disease and severe hepatic impairment. It has been associated with hepatotoxicity and cholestatic jaundice.
Patients who are at risk of respiratory depression should not take it. Likewise, people who drive or perform tasks that require mental alertness should avoid it as it causes drowsiness, memory loss, confusion, and impairment of physical and mental abilities.
Robaxin CVS effects:
Robaxin (Methocarbamol), apart from its central nervous system side effects, also has adverse cardiovascular effects. It commonly causes a slowing of the heart rate, lowering of the blood pressure, dizziness, postural hypotension, flushing, and may cause inflammation of the veins when injected intravenously.
Avoid Rapid IV injection
Common Brand Names:
It is available in 500 mg and 750 mg tablets. Common brand names include:
- Robaxin-500 mg,
- Robaxin-750 mg
Methocarbamol is the generic name of Robaxin and other brands. It is a centrally-acting muscle relaxant with sedative properties. It is available as injections and tablets. It is commonly used to treat acute lower back pain and other painful conditions caused by muscle spasm.