What the FDA labels say about taking them together
Metaxalone and morphine
Morphine's FDA label of July 13, 2026 names metaxalone in its drug interactions section; metaxalone's label does not name morphine itself but warns about its class (Opioids (opioid agonists)). The morphine label: “The concomitant use of opioids with other drugs that affect the serotonergic neurotransmitter system has resulted in serotonin syndrome.”
Check it at the source: the metaxalone FDA label on DailyMed (effective January 11, 2023, Amneal Pharmaceuticals of New York LLC); the morphine FDA label on DailyMed (effective July 13, 2026, Bryant Ranch Prepack).
What does the metaxalone label say about morphine?
Metaxalone's label does not name morphine itself; it warns about its class (Opioids (opioid agonists)):
Metaxalone label, warnings and precautionsCases of serotonin syndrome, a potentially life-threatening condition, have been reported during concomitant use of metaxalone (within the recommended dosage range) and other serotonergic drugs [see Drug Interactions (7) ] and with the use of metaxalone as the only serotonergic drug taken at a dosage higher than the recommended dosage [see Overdosage (10) ]. …
Metaxalone label, drug interactionsDiscontinue metaxalone if serotonin syndrome is suspected. Examples of serotonergic drugs include: selective serotonin reuptake inhibitors (SSRIs), serotonin and norepinephrine reuptake inhibitors (SNRIs), tricyclic antidepressants (TCAs), triptans, 5-HT3 receptor antagonists, opioids (particularly fentanyl, meperidine, and methadone), drugs that affect the serotonin neurotransmitter system (e.g., mirtazapine, trazodone, tramadol), monoamine oxidase (MAO) inhibitors (those intended to treat psychiatric disorders …
Source: FDA drug label (Metaxalone), Amneal Pharmaceuticals of New York LLC, effective January 11, 2023 (SPL set f06f2b96-3684-483c-89f7-6c9201fd4664), via openFDA; the full label on DailyMed
What does the morphine label say about metaxalone?
Morphine label, drug interactionsSerotonergic Drugs Clinical Impact: The concomitant use of opioids with other drugs that affect the serotonergic neurotransmitter system has resulted in serotonin syndrome. … neurotransmitter system (e.g., mirtazapine, trazodone, tramadol), certain muscle relaxants (i.e., cyclobenzaprine, metaxalone), monoamine oxidase (MAO) inhibitors (those intended to treat psychiatric disorders and also others, such as linezolid and intravenous methylene blue).
Source: FDA drug label (Ms Contin, NDA019516), Rhodes Pharmaceuticals LLC, effective August 14, 2026 (SPL set c354b3bf-86c0-4bb8-8b1f-be2164942698), via openFDA
Source: FDA drug label (Morphine sulfate), ANDA078761 (Sun Pharm Inds Ltd), as published by Bryant Ranch Prepack, effective July 13, 2026 (SPL set 595243fe-a4b1-479a-99d6-a084a4181891), via openFDA; the full label on DailyMed
These are the labels' own sentences. PlainMeds does not grade interactions; a pharmacist or prescriber can say what they mean for one person's medicines.
Both medicines on the record
| Metaxalone | Morphine | |
|---|---|---|
| Labeled for | Muscle spasms and spasticity | Pain |
| Boxed warning | No | Yes |
| Pharmacy cost (NADAC) | $0.452 per tablet 800 mg tablet | $0.226 per tablet 15 mg tablet |
| Shortage now | No current entry | In shortage |
Source: FDA NDC Directory, CMS NADAC of October 7, 2026 and the FDA Drug Shortage Database (FDA files October 8, 2026)
What next?
- Check more medicines together
- up to five at once