Commentary: Is Polyethylene Glycol Toxicity From Intravenous Methocarbamol Fact or Fiction?

Methocarbamol is an antispasmodic muscle relaxant and was the fourth most-prescribed muscle relaxant by volume in the United States in 2021. Intravenous (IV) methocarbamol contains the excipient, polyethylene glycol (PEG), which has been implicated in metabolic acidosis and nephrotoxicity. Intravenous methocarbamol was first approved by the US Food and Drug Administration in 1959 and at that time the IV methocarbamol prescribing information warned of PEG-associated adverse drug events in patients living with renal impairment; however, the manufacturer acknowledged data were lacking to objectively support this claim. Clinicians prescribing and dispensing IV methocarbamol may encounter the warning for PEG-associated metabolic acidosis and nephrotoxicity without knowing the potential risks, or lack thereof, supporting or disavowing this phenomenon. This commentary debates the merits supporting and arguments refuting PEG-associated metabolic acidosis and nephrotoxicity in patients treated with IV methocarbamol.

A Randomized, Double-Blind, Placebo-Controlled Trial of Naproxen With or Without Orphenadrine or Methocarbamol for Acute Low Back Pain.

STUDY OBJECTIVE: In US emergency departments (EDs), patients with low back pain are often treated with nonsteroidal anti-inflammatory drugs and muscle relaxants. We compare functional outcomes among patients randomized to a 1-week course of naproxen+placebo versus naproxen+orphenadrine or naproxen+methocarbamol. METHODS: This was a randomized, double-blind, comparative effectiveness trial conducted in 2 urban EDs. Patients presenting with acute, nontraumatic, nonradicular low back pain were enrolled. The primary outcome was improvement on the Roland-Morris Disability Questionnaire (RMDQ) between ED discharge and 1 week later. All patients were given 14 tablets of naproxen 500 mg, to be used twice a day, as needed for low back pain. Additionally, patients were randomized to receive a 1-week supply of orphenadrine 100 mg, to be used twice a day as needed, methocarbamol 750 mg, to be used as 1 or 2 tablets 3 times per day as needed, or placebo. All patients received a standardized 10-minute low back pain educational session before discharge. RESULTS: Two hundred forty patients were randomized. Baseline demographic characteristics were comparable. The mean RMDQ score of patients randomized to naproxen+placebo improved by 10.9 points (95% confidence interval [CI] 8.9 to 12.9). The mean RMDQ score of patients randomized to naproxen+orphenadrine improved by 9.4 points (95% CI 7.4 to 11.5). The mean RMDQ score of patients randomized to naproxen+methocarbamol improved by 8.1 points (95% CI 6.1 to 10.1). None of the between-group differences surpassed our threshold for clinical significance. Adverse events were reported by 17% (95% CI 10% to 28%) of placebo patients, 9% (95% CI 4% to 19%) of orphenadrine patients, and 19% (95% CI 11% to 29%) of methocarbamol patients. CONCLUSION: Among ED patients with acute, nontraumatic, nonradicular low back pain, combining naproxen with either orphenadrine or methocarbamol did not improve functional outcomes compared with naproxen+placebo.

Multimodal Drug Therapy and Physical Rehabilitation in the Successful Treatment of Capture Myopathy in a Lesser Flamingo (Phoeniconaias minor).

A wild-caught lesser flamingo (Phoeniconaias minor) from the Fort Worth Zoo (Fort Worth, TX, USA) presented with moderate lameness that progressed to the inability to stand 2 days after restraint and handling. Results of blood tests showed severely elevated creatine phosphokinase (CPK) and aspartate aminotransferase (AST) activities, confirming suspected capture myopathy. Intensive supportive therapy, consisting of intravenous fluids and muscle relaxants, along with physical rehabilitation therapy, nutritional support, and anxiolytics, were instituted to aid in relaxation and muscle regeneration. After 2 weeks of intensive therapy, the bird showed substantial improvement and could remain standing throughout the day after being assisted to a standing position. By day 23, the bird was able to stand independently and walk completely unassisted, with no discernible lameness. The bird has subsequently remained healthy since it was returned to the flock approximately 27 days after it was first presented for treatment. Although anecdotal communications of successful treatment of this condition in flamingos exist, this is the first report, to our knowledge, that describes in detail the successful treatment of capture myopathy in any flamingo species. Success in this case is attributed to the combination of early fluid and drug therapy, intensive physical rehabilitation therapy, and anxiolytics to counteract the hyperexcitable nature of this wild-caught bird.

Pharmacokinetics of methocarbamol and phenylbutazone in exercised Thoroughbred horses.

Methocarbamol (MCBL) is commonly used in performance horses for the treatment of skeletal muscle disorders. Current regulatory recommendations for show horses and racehorses are based on a single oral dose of 5 g, although doses in excess of this are often administered. The goal of the current study was to characterize the disposition of MCBL following higher dose administration and administration in combination with another commonly used drug in performance horses, phenylbutazone (PBZ). Exercised Thoroughbred horses were administered various doses of MCBL as a sole agent and MCBL in combination with PBZ. Blood samples were collected at various times, concentrations of MCBL and PBZ measured using LC-MS/MS and pharmacokinetic parameters calculated using compartmental analysis. Following administration of 15 g of MCBL, either as part of a single- or multiple-dose regimen, a number of horses exceeded the Association of Racing Commissioners International and the United States Equestrian Federation's recommended regulatory threshold at the recommended withdrawal time. There was not a significant difference between horses that received only MCBL and those that received MCBL and PBZ. Results of the current study support an extended withdrawal guideline when doses in excess of 5 g are administered.

The pharmacokinetics of methocarbamol and guaifenesin after single intravenous and multiple-dose oral administration of methocarbamol in the horse.

A simple LC/MSMS method has been developed and fully validated to determine concentrations and characterize the concentration vs. time course of methocarbamol (MCBL) and guaifenesin (GGE) in plasma after a single intravenous dose and multiple oral dose administrations of MCBL to conditioned Thoroughbred horses. The plasma concentration-time profiles for MCBL after a single intravenous dose of 15 mg/kg of MCBL were best described by a three-compartment model. Mean extrapolated peak (C0 ) plasma concentrations were 23.2 (± 5.93) µg/mL. Terminal half-life, volume of distribution at steady-state, mean residence time, and systemic clearance were characterized by a median (range) of 2.96 (2.46-4.71) h, 1.05 (0.943-1.21) L/kg, 1.98 (1.45-2.51) h, and 8.99 (6.68-10.8) mL/min/kg, respectively. Oral dose of MCBL was characterized by a median (range) terminal half-life, mean transit time, mean absorption time, and apparent oral clearance of 2.89 (2.21-4.88) h, 2.67 (1.80-2.87) h, 0.410 (0.350-0.770) h, and 16.5 (13.0-20) mL/min/kg. Bioavailability of orally administered MCBL was characterized by a median (range) of 54.4 (43.2-72.8)%. Guaifenesin plasma concentrations were below the limit of detection in all samples collected after the single intravenous dose of MCBL whereas they were detected for up to 24 h after the last dose of the multiple-dose oral regimen. This difference may be attributed to first-pass metabolism of MCBL to GGE after oral administration and may provide a means of differentiating the two routes of administration.

Adjunctive therapy with intravenous lipid emulsion and methocarbamol for permethrin toxicity in 2 cats.

OBJECTIVE: To describe the use of an intravenous lipid emulsion (ILE) as an adjunctive therapy in 2 cats with permethrin toxicity. CASE SERIES SUMMARY: Two cats that presented with severe permethrin toxicity were treated with ILE as part of their treatment regimens. Both cats improved dramatically following therapy with decontamination, ILE, methocarbamol, and supportive care. NEW OR UNIQUE INFORMATION PROVIDED: This is the first reported use of ILE as an adjunctive treatment for cats with permethrin toxicity. Outcome was favorable in both cats and no adverse effects were noted from the ILE.

Effect of preoperative intravenous methocarbamol and intravenous acetaminophen on opioid use after primary total hip and knee replacement.

Between 2010 and 2011, a perioperative pain protocol for primary total hip and knee replacement at one Florida medical center replaced preoperative oral analgesics with intravenous methocarbamol and intravenous acetaminophen. This is a retrospective cohort study of 300 patients, with 150 patients using the new pain protocol and 150 patients using a 2008 pain protocol that did not include these medications. The 2 cohorts were similar in patient gender, age, and body mass index. Opioid consumption was evaluated for a period of 48 hours after incision and was divided into 3 separate time intervals, as well as total 48-hour consumption. Mean opiate use decreased significantly from 2008 to 2011 in all time intervals and total consumption (7.5±3.4 mg to 6.1±3.0 mg; P<.01). Subgroup analysis suggested that changes to the hip protocol were responsible for decreased opioid use in the operating room and the postanesthesia care unit, and changes to the knee protocol were responsible for decreased opioid use on the hospital floor and total consumption. The difference between the 2 protocol groups was not due to differences in individual surgeon practice patterns. Physical therapy progress of knee flexion, average walking distance, and maximum walking distance were significantly improved. Hospital discharge was shorter in the 2011 group (4.0±1.1 days in 2008 group and 3.6±1.0 days in 2011 group). This study shows significant improvement in patient care from 2008 to 2011 that is at least partially due to the change to the use of preoperative intravenous methocarbamol and intravenous acetaminophen.

Methocarbamol suspension for the treatment of rhabdomyolysis in equines.

Rhabdomyolysis in equines occurs in horses due to physical overexertion or underlying pathologic myopathy. Methocarbamol is a muscle relaxant that can be used in equines to treat symptoms associated with Rhabdomyolysis. Methocarbamol is available as a solution for injection but is not commercially available as an oral suspension. This article focuses on the treatment of Tying-up caused by overexertion, and details the treatment of Rhabdomyolysis with an oral suspension that was prepared for a veterinarian by a compounding pharmacist.

Acetaminophen toxicity with concomitant use of carbamazepine.

Acetaminophen is a widely used analgesic that can cause acute liver failure when consumed above a maximum daily dose. Certain patients may be at increased risk of hepatocellular damage even at conventional therapeutic doses. We report a case of a 34-year-old man on carbamazepine for complex partial seizures who developed acute liver and renal failure on less than 2.5 grams a day of acetaminophen. This raises caution that patients on carbamazepine should avoid chronic use of acetaminophen, and if required use at lower doses with vigilant monitoring for signs of liver damage.

Efficacy and safety of sodium hyaluronate in the treatment of Wilkes stage II disease.

PURPOSE: To show whether an intra-articular (IA) infiltration of 1 mL sodium hyaluronate (SH) into the temporomandibular joint (TMJ) would significantly reduce pain and improve joint function in Wilkes stage II disease, compared with the oral administration of a combination of methocarbamol and paracetamol. PATIENTS AND METHODS: Forty-one patients with Wilkes stage II disease were selected and randomly assigned to 2 groups. The experimental group received 1 IA infiltration of SH with assessments at days 14, 28, 56, and 84. The control group was given 2 tablets of a combination of methocarbamol 380 mg and paracetamol 300 mg every 6 hours for 4 weeks, with assessments at days 14 and 28. RESULTS: Forty-one patients were randomized into the study (SH: 20 patients, control drug: 21 patients). A statistically significant difference (P < . 05) was detected in favor of the SH group from day 56 onward for TMJ pain at rest, from day 14 onward for pain on jaw opening, and at days 28 and 56 for pain on mastication. The TMJ function was statistically significantly (P < .05) better in the test group at all follow-up visits. The global evaluation of efficacy by both, the patients and investigators, was better for the test group. No adverse reactions were detected with SH. CONCLUSIONS: An IA infiltration of SH showed better efficacy in reducing pain and improving joint function in Wilkes stage II disease, compared with the oral administration of methocarbamol-paracetamol tablets.

Pain reduction in breast augmentation using methocarbamol.

Submuscular placement of breast implants produces significant postoperative pain and discomfort. The standard use of narcotics alone does not optimize pain reduction. Methocarbamol was used intraoperatively and postoperatively in 62 patients undergoing manipulation of the pectoralis major associated with breast implant surgery. Significant pain relief was achieved.

The pharmacology and pharmacokinetics of high-dose methocarbamol in horses.

The haemodynamic, respiratory and behavioural effects and pharmacokinetics of methocarbamol were studied in eight healthy, adult horses after intravenous (i.v.) and oral administration of large dosages. Heart rate, cardiac output, mean pulmonary arterial blood pressure, systolic, diastolic and mean aortic blood pressure, respiratory rate and arterial blood gases did not change after either i.v. (30 mg/kg bodyweight [bwt]) or oral (50 and 100 mg/kg bwt) dosages of methocarbamol. Mild to moderate depression was observed in five of eight horses administered i.v. methocarbamol, and in all horses administered oral methocarbamol. Plasma methocarbamol concentration declined very rapidly during the initial or rapid disposition phase after i.v. administration; the terminal elimination half-life ranged from 59 to 90 mins. Peak plasma methocarbamol concentrations following oral administration occurred within 15 to 45 mins and oral bioavailability ranged from 50.7 to 124 percent.

Pharmacokinetics and bioavailability of methocarbamol in rats.

In a pharmacokinetic study, 15, 30, 60, and 150 mg kg-1 intravenous and oral doses of methocarbamol were administered to rats. Differences observed in plasma clearance values, i.e. 0.0203, 0.0156, 0.0123, and 0.0085 1 kg-1 min-1 for 15, 30, 60, and 150 mg kg-1, respectively, suggested a dose-dependent pharmacokinetic behaviour of the drug. Elimination according to a biocompartmental open model and Michaelis-Menten kinetics fits the plasma level data. Estimated Km and Vmax values were 38.49 +/- 3.71 mg l-1 and 1.24 +/- 0.06 mg l-1 min-1, respectively. After oral administration of 15, 30, and 60 mg kg-1 the peak plasma levels were reached earlier. The tmax values were 6, 6, and 10 min, respectively. After 150 mg kg-1 oral doses, peak plasma levels were reached later (tmax = 150 min). Estimated bioavailability ranged between 77 and 112 per cent.

[Electroencephalographic effects of chlorphenesin carbamate, a new central muscle relaxant, in rabbits (author's transl)].

Electroencephalographic (EEG) effects of chlorphenesin carbamate were investigated in rabbits with chronic electrode implants, and compared with those of chlormezanone and methocarbamol. Chlorphenesin carbamate (50 mg/kg i.v., 100 mg/kg i.d.) induced a drowsy pattern of spontaneous EEG consisting of high voltage slow waves in the cortex and amygdala, and desynchronization of hippocampal theta waves. Chlormezanone also elicited similar EEG changes but such were much more potent than chlorphenesin carbamate. Methocarbamol showed no effect on spontaneous EEG. Chlorphenesin carbamate caused sedation in this period and muscle relaxation was more potent than that of chlormezanone. The EEG arousal response to auditory stimulation and to electric stimulation of the posterior hypothalamus, centromedian thalamus and mesencephalic reticular formation was slightly depressed by chlorphenesin carbamate. Chlorphenesin carbamate, as with chlormezanone, markedly depressed the limbic afterdischarges elicited by hippocampal stimulation. These EEG effects of chlorphenesin carbamate were qualitatively similar to but much weaker than those of chlormezanone, whereas the muscle relaxant effect of chlorphenesin carbamate was more potent than that of chlormezanone.

Tetanus: conservative management made easier by combination of muscle relaxants.

Conservative management of 11 consecutive cases of tetanus with diazepam in heavy doses and methocarbamol is described. All drugs were administered intravenously through a cut-down. Patients were nursed near the nurses' station for the first 3 to 5 days when fatal complications and very severe spasms are most likely to occur. There was minimal interference with the patients and a reasonable reduction in the duration of significant spasms, and complications were minimized. No neonates were included. Severity of tetanus cannot be determined only by consideration of onset of symptoms before admission. It is suggested that a combination of muscle relaxants without heavy sedation may produce an improvement in the results of conservative management of clinical tetanus.