Methocarbamol Use Associated With Higher Postoperative Pain Burden, Increased Opioid Doses

The use of methocarbamol for postoperative pain management has been linked to heightened pain burden and increased opioid dose requirements, according to study findings published in The Clinical Journal of Pain.  

In a retrospective cohort analysis, researchers collected data from adult patients who underwent in-patient surgery involving the musculoskeletal system. The surgeries were performed under general anesthesia, regional anesthesia, or a combination of both. Linear regression was used to assess pain and quantile regression was used to assess opioid dosing. The researchers used propensity score-weighted regression models, standardized mortality ratio (SMR) weights, and post-hoc analysis to analyze the data.    

Primary outcome focused on acute pain intensity in the first 48 hours postoperatively. Acute pain intensity was measured using the time-weighted average (TWA) pain score. To evaluate postoperative pain, nurses conducted assessments every 15 minutes for the first 2 hours, followed by assessments every 4 hours until 48 hours postoperatively. Secondary outcome included opioid dose requirements in morphine milligram equivalents during the first 48 hours postoperatively and the length of hospital stay.

Researchers recruited a total of 9089 patients who underwent surgery involving the musculoskeletal system and required a postoperative hospital admission lasting at least 48 hours. Among them, 704 patients received methocarbamol within the first 48 hours postoperatively, while 8385 patients did not receive methocarbamol. Patients were not receiving other muscle relaxants besides methocarbamol.

The mean [SD] postoperative 48-hour TWA pain scores were 5.5 [1.7] for patients who received methocarbamol and 4.3 [2.1] for non-methocarbamol patients. The postoperative 48-hour opioid dose requirements were higher for patients who received methocarbamol vs patients who did not receive methocarbamol (MME, 276 vs 190; interquartile range, 170-347 vs 60-248). After adjusting for potential confounders using multivariable regression, the study found that patients who received methocarbamol postoperatively experienced a 0.97-point higher mean postoperative TWA pain score (95% CI, 0.83-1.11; P < .001) and a 93.6-MME higher median postoperative opioid dose requirement (95% CI, 79.9-107.4; P < .001) compared with patients who did not receive methocarbamol.

The length of hospital stay was similar in the methocarbamol and non-methocarbamol groups (mean, 5 days [IQR, 4-7] vs 5 days [IQR, 4-8], respectively).

The study authors ran a sensitivity analysis and excluded observations with extreme propensity score weights. While small changes in point estimates and confidence intervals were noted in patients who received methocarbamol treatment, it did not change the study’s conclusions. 

Limitations of the study include the absence of pain scores and opioid doses recorded prior to the initial administration of methocarbamol in the outcomes, potentially influencing the true effect size of methocarbamol on the outcome. Additionally, the study is subject to residual confounding, and only randomized controlled trials can accurately quantify the true effect.

Overall, study authors concluded that “postoperative methocarbamol was associated with significantly increased acute postoperative pain burden and opioid dose requirements.”