Muscle Relaxant side effects compared: 5 drugs, 62,758 FDA adverse event reports
Members
| Drug | FAERS reports | Serious | Death outcome | Boxed warning | Top labeled signals | Unlabeled (unassessed) |
|---|---|---|---|---|---|---|
| Methocarbamol | 22,550 | 62% | 1,735 | – | – | Nephrogenic anaemia; Intervertebral disc degeneration |
| Carisoprodol | 18,961 | 69% | 4,011 | – | Multiple drug overdose; Multiple drug overdose intentional; Multiple drug overdose accidental | Respiratory arrest; Drug toxicity |
| Methocarbamol | 16,630 | 63% | 1,398 | – | – | Chronic kidney disease; End stage renal disease |
| Metaxalone | 3,321 | 62% | 603 | – | Serotonin syndrome; Respiratory depression | Toxicity to various agents; Chronic kidney disease |
| Chlorzoxazone | 1,296 | 65% | 130 | – | – | Chronic kidney disease; Emotional distress |
Class-wide signals (flagged in at least 3 of 5 members)
| Reaction | Members flagged | Median PRR | Reports (sum) | In label |
|---|---|---|---|---|
| Chronic kidney disease-mineral and bone disorder | 3/5 | 41.1 | 116 | 0/3 members' labels |
| Hyperparathyroidism secondary | 3/5 | 23.7 | 155 | 0/3 members' labels |
| Rebound acid hypersecretion | 3/5 | 23.6 | 195 | 0/3 members' labels |
| Nephrogenic anaemia | 3/5 | 23.4 | 294 | 0/3 members' labels |
| Lumbar radiculopathy | 3/5 | 17.0 | 53 | 0/3 members' labels |
| Pre-existing condition improved | 3/5 | 12.8 | 223 | 0/3 members' labels |
| End stage renal disease | 3/5 | 12.3 | 220 | 0/3 members' labels |
| Chronic kidney disease | 3/5 | 10.7 | 815 | 0/3 members' labels |
Member-specific signals (flagged in exactly one drug)
| Reaction | Drug | Reports | PRR | Label screen |
|---|---|---|---|---|
| Multiple drug overdose | Carisoprodol | 217 | 49.8 | Labeled adverse reaction |
| Accidental death | Carisoprodol | 68 | 42.1 | Not in label — unassessed |
| Multiple drug overdose intentional | Carisoprodol | 101 | 35.3 | Labeled (related wording) |
| Drug abuser | Carisoprodol | 130 | 23.5 | Not in label — unassessed |
| Serotonin syndrome | Metaxalone | 59 | 19.5 | Labeled adverse reaction |
| Exercise tolerance decreased | Methocarbamol | 53 | 11.8 | Not in label — unassessed |
| Blood pressure inadequately controlled | Methocarbamol | 55 | 11.5 | Not in label — unassessed |
| Synovial cyst | Methocarbamol | 52 | 10.5 | Not in label — unassessed |
| Toxicity to various agents | Metaxalone | 164 | 6.2 | Not in label — unassessed |
| Renal failure | Metaxalone | 107 | 5.0 | Not in label — unassessed |
Common questions
Which muscle relaxant has the most FDA adverse event reports?
Methocarbamol has the most FAERS co-reports in this class (22,550), followed by Carisoprodol (18,961). Report volume tracks how widely and how long a drug has been used, and how much attention it has had, far more than its risk.
Which side effects are shared across muscle relaxant drugs?
Reaction terms flagged in at least half of the 5 members include chronic kidney disease-mineral and bone disorder, hyperparathyroidism secondary, rebound acid hypersecretion, nephrogenic anaemia, lumbar radiculopathy. A term shared across a class usually reflects the shared mechanism (or the shared patient population) rather than one product.
Does a FAERS signal mean the drug causes the reaction?
No. These are disproportionality signals from voluntary reports with no denominator. Each is screened against the FDA label (known vs unlabeled) on the drug pages; causality needs temporality, dose-response, plausibility and trial or cohort evidence.
Sources: openFDA FAERS pair statistics (OpenPV build 2026-09-14), FDA labels via openFDA, seriousness and death counts from openFDA count queries. Generated 2026-10-08. Not medical advice.