Methocarbamol – ASA – Codeine Phosphate
Skeletal Muscle Relaxant – Analgesic
Action And Clinical Pharmacology: The mechanism of action of methocarbamol has not been established, but may be due to general CNS depression. It has no direct action on the contractile mechanism of striated muscle, the motor end plate or the nerve fibre. Methocarbamol is metabolized to yield a dealkylated and a hydroxylated product. These 2 metabolites are found primarily as glucuronide and sulfate conjugates. Based on elimination of radioactivity, the half-life of methocarbamol and its metabolites is about 2 hours. Animal studies reveal that methocarbamol crosses the placental barrier and blood-brain barrier.
ASA interferes with the production of prostaglandins in various organs and tissues through acetylation and inactivation of the enzyme cyclooxygenase. The main action of the drug is thought to be peripheral; however, it may have similar activity in the CNS. The reduction in tissue levels of prostaglandins may be responsible for the analgesic and anti-inflammatory effects of the drug. ASA is most effective against pain of low to moderate intensity associated with inflammation.
Codeine is readily absorbed from the gastrointestinal tract, and a therapeutic dose reaches peak analgesic effectiveness in about 2 hours and persists for 4 to 6 hours. Oral codeine (60 mg) given to healthy males has been shown to achieve peak blood levels of 0.016 mg/100 mL at approximately 1 hour post-dose. The codeine plasma half-life for a 60 mg oral dose is about 2.9 hours. Blood levels causing CNS depression begin at 0.05 to 0.19 mg/100 mL.
Indications And Clinical Uses: Relief of acute episodes of severe pain associated with skeletal muscle spasm: acute torticollis, acute strains and sprains, acute low back pain, acute tenosynovitis, ankle sprain, fracture, trauma, acute bursitis, acute myositis, whiplash injury.
Contra-Indications: Hypersensitivity to methocarbamol, ASA, or codeine.
Patients who have had a bronchospastic reaction, generalized urticaria, angioedema, severe rhinitis, laryngeal edema or shock precipitated by ASA or nonsteroidal anti-inflammatory drugs. Some patients sensitive to ASA, may be cross-sensitive to other nonsteroidal anti-inflammatory drugs as well as tartrazine dye. Patients with asthma associated nasal poylps have an increased risk of sensitivity to ASA.
Do not use in patients with active peptic ulcer.
Precautions: General: The administration of opioids may obscure the diagnosis or clinical course in patients with acute abdominal conditions. In the presence of head injury, other intracranial lesions or a pre-existing increase in intracranial pressure, the respiratory depressant effects of opioids and their capacity to elevate cerebrospinal fluid pressure may be markedly exaggerated. Furthermore, narcotics produce adverse reactions which may obscure the clinical course of patients with head injuries.
Occupational Hazards: Robaxisal-C may impair the mental and/or physical abilities required for the performance of potentially hazardous tasks such as driving a car or operating machinery. The patient using this drug should be cautioned accordingly.
Drug Interactions: Patients receiving other opioid analgesics, general anesthetics, phenothiazines, tranquilizers, sedative-hypnotics or other CNS depressants (including alcohol) concomitantly with Robaxisal-C may exhibit an additive CNS depression. When such combined therapy is contemplated, the dose of one or both agents should be reduced.
The use of MAO inhibitors or tricyclic antidepressants with codeine preparations may increase the effect of either the antidepressant or codeine. The concurrent use of anticholinergics with codeine may produce paralytic ileus.
Methocarbamol may cause a color interference in certain screening tests for 5-hydroxyindoleacetic acid (5-HIAA) and vanillylmandelic acid (VMA).
Carcinogenesis, Mutagenesis: Long-term studies in animals have not been performed to evaluate carcinogenic or mutagenic potential.
Pregnancy: There are no adequate and well-controlled studies in pregnant women. This product should be used during pregnancy only when in the judgment of the physician the potential benefits outweigh the potential hazards.
Adverse Reactions: Methocarbamol: The most common complaints to methocarbamol are drowsiness, nausea and dizziness or lightheadedness (seen in approximately 4 to 5 % of patients). The following reactions have been associated with the drug, some of them rarely; in some instances, causal relationships have not been established: headache, nasal congestion, blurred vision, rash, pruritus and urticaria.
ASA: Gastrointestinal: ulcer, hemorrhage, dyspepsia, heartburn, epigastric distress, nausea, vomiting, diarrhea, abdominal pain may occur with increasing incidence at higher dosages.
Hepatic: Reversible hepatotoxicity particularly in patients with juvenile rheumatoid arthritis and systemic lupus erythematosus has been reported rarely.
Otic: Tinnitus and hearing loss, usually completely reversible, may occur in patients receiving large doses of ASA or with long-term use and are dose related.
Skin: Skin eruptions and lesions have been reported. Stevens-Johnson’s syndrome has rarely been associated with ASA.
Chronic salicylate intoxication may result from high doses or from prolonged therapy with high doses. Tinnitus and hearing loss are the most frequent signs of chronic intoxication. Other manifestations such as dimness of vision, headache, dizziness, mental confusion, drowsiness, sweating, thirst, hyperventilation, tachycardia, nausea, vomiting and sometimes diarrhea may occur.
Codeine: Drug Abuse and Dependence: Codeine can produce drug dependence of the morphine type, and therefore has the potential for being abused. Psychic dependence, physical dependence and tolerance may develop upon repeated administration of this drug, and it should be prescribed and administered with the same degree of caution appropriate to the use of other oral opioid-containing medications.
Symptoms And Treatment Of Overdose: Symptoms: Methocarbamol: No deaths or major toxicity have been reported from overdosage with methocarbamol, administered parenterally or orally. One adult survived the deliberate ingestion of 22 to 30 g of methocarbamol without serious toxicity. Another survived 30 to 50 g. The principal symptom was drowsiness in both cases.
Codeine: Respiratory depression (reduced respiratory rate and/or tidal volume; Cheyne-Stokes respiration; cyanosis), extreme somnolence progressing to stupor or coma, skeletal muscle flaccidity, cold or clammy skin, and sometimes hypotension and bradycardia. Severe overdosage may result in apnea, circulatory collapse, cardiac arrest and death. Miosis can be one characteristic of morphine derivative overdose. Mydriasis can take place in terminal narcosis, severe hypoxia or as a toxic effect of pethidine or its congeners.
ASA: Symptoms of acute toxicity with ASA may occur with doses greater than 150 mg/kg. Doses greater than 500 mg/kg are potentially fatal. Acid-base and electrolyte disturbances, dehydration, hyperpyrexia, hyperglycemia or hypoglycemia are the principal physiologic manifestations of acute ASA toxicity. Other symptoms of toxicity include burning pain in the mouth or throat, dizziness, tinnitus and sweating. In more severe cases, presence of CNS symptoms such as lethargy, disorientation, or confusion may be a predictor for the development of pulmonary edema. Coma and convulsions may be delayed for 24 to 48 hours. Cardiac arrhythmias have been reported. Bleeding disorders, cerebral edema, oliguria are also possible.
Treatment: Gastric lavage, administration of activated charcoal. If respiratory depression occurs, give attention to the re-establishment of respiratory exchange by ventilation and administration of a narcotic antagonist, e.g., naloxone.
Dosage: Adults: 1 or 2 tablets 3 or 4 times a day.
Availability And Storage: Robaxisal-C 1/8: Each yellow and white tablet, yellow layer monogrammed “WR”, contains: methocarbamol 400 mg, ASA 325 mg, codeine phosphate 8 mg. Nonmedicinal ingredients: cornstarch, D&C Yellow No. 10, FD&C Yellow No. 6, magnesium stearate, povidone, sodium starch glycolate and stearic acid. Energy:
Robaxisal-C 1/4: Each orange and white tablet, orange layer monogrammed “WR”, contains: methocarbamol 400 mg, ASA 325 mg and codeine phosphate 16.2 mg. Nonmedicinal ingredients: cornstarch, FD&C Yellow No. 6, magnesium stearate, povidone, sodium starch glycolate and stearic acid. Energy:
Robaxisal-C 1/2: Each coral and white caplet, coral layer monogrammed “WR”, contains: methocarbamol 400 mg, ASA 325 mg, codeine phosphate 32.4 mg. Nonmedicinal ingredients: cellulose, cornstarch, FD&C Red No. 2, FD&C Yellow No. 6, magnesium stearate, povidone, sodium starch glycolate and stearic acid. Energy:
Store at room temperature (15 to 30°C).
ROBAXISAL®-C Whitehall-Robins Methocarbamol – ASA – Codeine Phosphate Skeletal Muscle Relaxant – Analgesic