Drug Interaction Report

Chlorzoxazone and Morphine Injection: Interaction Details

Name: Chlorzoxazone and Morphine Injection Interaction
Creator: Dr. Brian Staiger, PharmD, BCPS

A pharmacist-reviewed analysis of the Chlorzoxazone and Morphine Injection interaction: the documented clinical effects, severity rating, onset, mechanism, and management considerations, with cited references throughout.

AI-assisted, pharmacist-reviewed · Source data updated Aug 8, 2026 · Sources: FDA labeling, DDInter 2.0, cited literature

Chlorzoxazone

Lorzone

Morphine Injection

Astramorph® PF Duramorph Duramorph® Infumorph Infumorph® Kadian Mitigo MS Contin

Medically reviewed by

Dr. Brian Staiger, PharmD, BCPS

Updated Aug 8, 2026

Editorial policy · Data sources

Interaction severity

Major

Potentially serious — often needs a change or close monitoring.

Minor

Moderate

Major

Contraindicated

How we grade severity & evidence

Severity levels

Contraindicated: These should generally not be used together.
Major: Potentially serious — often needs a change or close monitoring.
Moderate: Can be significant — usually manageable with monitoring.
Minor: Usually limited clinical impact.

Evidence grades

Established: Well documented — supported by controlled studies or strong clinical data.
Probable: Good supporting evidence, though not definitively proven.
Suspected: Some evidence suggests this interaction, but it is not well established.
Possible: Limited or conflicting evidence; the interaction may occur.
Theoretical: Predicted from the drugs' pharmacology; not yet confirmed in people.

Ratings come from the documented interaction literature and are reviewed by a pharmacist. They describe the documented risk of the combination, not what will necessarily happen to you — your dose, timing, and health picture all matter.

Of 35 documented Chlorzoxazone interactions, 31 are rated major — including this one.

Worried about symptoms right now? Contact your pharmacist or prescriber, or call Poison Control at 1-800-222-1222 (US). Call 911 for an emergency.

Onset

unspecified

Evidence

theoretical

Severity

Major

What happens

Increased risk of profound sedation, hypotension, coma and respiratory depression

Interaction Deep Dive

Concomitant use of morphine with skeletal muscle relaxants may enhance the neuromuscular blocking action of muscle relaxants and produce an increased degree of respiratory depression. Monitor patients for signs of respiratory depression that may be greater than otherwise expected and decrease the dosage of meperidine and/or the muscle relaxant as necessary. Additionally, concomitant use of morphine and CNS depressants may increase the risk of profound sedation, hypotension, coma, opioid overdose and respiratory depression. Reserve the concomitant use of morphine and CNS depressants to patients for whom alternatives are inadequate. If concomitant use is necessary, use the lowest dose and shortest duration necessary to achieve treatment goals. Consider using a lower dose of morphine or the CNS depressant and closely monitor for sedation and respiratory depression. The risk of opioid overdose is increased with concurrent use of morphine with CNS depressants. Consider prescribing naloxone for the emergency treatment of opioid overdose⁴¹³².

Why it happens (mechanism)

Additive CNS depression; enhanced neuromuscular blockade

Common questions

Can I take Chlorzoxazone and Morphine Injection together?

Increased risk of profound sedation, hypotension, coma and respiratory depression Always confirm with your pharmacist or prescriber before making any change.

How serious is the Chlorzoxazone and Morphine Injection interaction?

It is rated major. Potentially serious — often needs a change or close monitoring.

How quickly could this interaction happen?

The documented onset is "unspecified". The timing of this interaction is not well characterized.

How strong is the evidence for this interaction?

The evidence is graded "theoretical". Predicted from the drugs' pharmacology; not yet confirmed in people.

Questions for your pharmacist

Does my dose of Chlorzoxazone or Morphine Injection need adjusting while I take them together?
What symptoms should prompt me to call you or my prescriber right away?
Does the timing of my doses matter for this combination?
Is there a safer alternative to one of these medications for me?

References (4)

Product Information: MS CONTIN(R) oral extended-release tablets, morphine sulfate oral extended-release tablets. Purdue Pharma LP (per FDA), Stamford, CT, 2023. DailyMed
Product Information: INFUMORPH intrathecal, epidural injection, morphine sulfate intrathecal, epidural injection. Hikma Pharmaceuticals USA Inc (per FDA), Berkeley Heights, NJ, 2023. DailyMed
Product Information: Morphine sulfate oral solution, morphine sulfate oral solution. Hikma Pharmaceuticals USA Inc (per FDA), Berkeley Heights, NJ, 2023. DailyMed
Product Information: DURAMORPH intravenous, epidural, intrathecal injection, morphine sulfate intravenous, epidural, intrathecal injection. Hikma Pharmaceuticals USA Inc (Per FDA), Berkeley, NJ, 2023. DailyMed

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Chlorzoxazone

Keep reading about Chlorzoxazone

All Chlorzoxazone interactions (35)

Also serious with: Nifedipine Mercaptopurine Colchicine Azathioprine

Morphine Injection

Keep reading about Morphine Injection

All Morphine Injection interactions (437) Morphine Injection drug information

Also serious with: Rasagiline Procarbazine Phenelzine Ozanimod

Beyond drug–drug

These medications also interact with supplements

Prescription drugs aren't the whole picture — herbal and dietary supplements can interact with them too. From the evidence-graded Natural Medicines database:

Chlorzoxazone + Activated Charcoal Procaine Coffee Charcoal Diosmin European Mandrake Evodia Glucomannan Guar Gum + 18 more

major · moderate · minor — check everything you take with our drug–supplement interaction checker.

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A message from HelloPharmacist

About our interaction database: depth, not breadth

HelloPharmacist doesn't license one of the big commercial interaction databases, so what you'll find here is not a complete list of every known drug interaction. Instead, each entry is researched, analyzed, and written up individually — documented from the published literature, reviewed by a pharmacist, and cited so you can check every source yourself.

If a combination isn't listed here, it means we haven't documented it yet — never that it's safe. For screening a full medication list, we recommend pairing this site with a comprehensive tool like the Drugs.com Interaction Checker and, most importantly, a review by your own pharmacist. For a deep, plain-English, fully sourced explanation of a specific documented interaction — that's exactly what we built this for.

This information is for education, not a substitute for professional medical advice. Do not start, stop, or change any medication without talking to your pharmacist or prescriber.