Short answer: Cyclobenzaprine (Flexeril) is a prescription muscle relaxant for short-term relief of muscle spasm. It is not a controlled substance. Chemically related to tricyclic antidepressants, it often causes drowsiness and dry mouth and can cause serotonin syndrome with some antidepressants.
Key takeaways
- What it is: Cyclobenzaprine is a centrally acting muscle relaxant (tricyclic-like) and not a federally controlled substance.
- Main uses: Muscle spasm (short term).
- Timing: effects start in about 1 hour and last 12 to 24 hours.
- Most dangerous mix: MAO inhibitors (within 14 days), which can cause seizures, high fever, death.
- Naloxone: it does not reverse this drug, but give it whenever opioids could be involved.
- Dependence risk: Low.
- Legal route: prescription only, filled by a licensed pharmacy.
What is cyclobenzaprine?
Cyclobenzaprine relieves muscle spasm from strains, sprains, and back pain, alongside rest and physical therapy. It is approved for 2 to 3 weeks because evidence for longer use is lacking.
It works in the brainstem rather than directly on muscles, and its structure is close to the antidepressant amitriptyline.
Quick facts
| Fact | Cyclobenzaprine |
|---|---|
| Generic name | Cyclobenzaprine hydrochloride |
| Brand names | Flexeril (discontinued brand), Fexmid, Amrix (extended release) |
| Drug class | Centrally acting skeletal muscle relaxant |
| US legal status | Prescription only; not controlled |
| Approved uses | Muscle spasm from acute, painful musculoskeletal conditions, for 2 to 3 weeks |
| Forms | Tablets (5, 7.5, 10 mg); extended-release capsules (15, 30 mg) |
| Onset | About 1 hour |
| Half-life | About 18 hours |
How long does cyclobenzaprine last?
- Starts working: About 1 hour
- Effects last: 12 to 24 hours
- Half-life: About 18 hours
It is approved for 2 to 3 weeks.
Timing varies with the dose, the form, how it is taken, liver and kidney function, age, and other drugs taken at the same time.
What are the biggest risks of cyclobenzaprine?
Common side effects include drowsiness, dry mouth, dizziness, fatigue, and constipation.
Heart: like tricyclics it can cause fast or irregular heart rhythms. It should not be used after a recent heart attack, with heart block, arrhythmias, heart failure, or an overactive thyroid.
Older adults: it is on the list of medicines to avoid in older adults because of confusion and falls.
What should you never mix with cyclobenzaprine?
| Combined with | What happens | Label guidance |
|---|---|---|
| MAO inhibitors (within 14 days) | Seizures, high fever, death | Contraindicated |
| SSRIs, SNRIs, tramadol, bupropion, triptans | Serotonin syndrome | Use caution |
| Alcohol, opioids, benzodiazepines | Additive sedation and slowed breathing | Avoid or use caution |
| Anticholinergic drugs | Urinary retention, confusion, overheating | Use caution |
Check any combination with the DoseUses combination checker or ask a pharmacist.
What are the signs of a cyclobenzaprine overdose, and what should you do?
Overdose causes drowsiness, fast heartbeat, agitation, hallucinations, and, at high doses, dangerous heart rhythms, seizures, and coma, similar to tricyclic antidepressant poisoning.
If someone may have taken too much
- Call 911 for fainting, a seizure, chest pain, a racing or irregular heartbeat, confusion, or if the person cannot be woken.
- If the person collapses and is not breathing normally, start CPR and use an AED if one is available.
- Give naloxone if opioids could be involved. It is safe even if they are not.
- For advice when the person is awake and stable, call Poison Help at 1-800-222-1222 (US, free, 24 hours). Bring the product packaging to the hospital.
What is cyclobenzaprine withdrawal like, and how long does it last?
Cyclobenzaprine does not usually cause dependence. Stopping after long-term daily use can cause temporary discomfort.
- Days 1 to 3Possible symptomsNausea, headache, and feeling unwell.
- Within a weekResolvesSymptoms are usually mild and short-lived.
- Ongoing painReassessExercise and physical therapy help more over time.
See the withdrawal timelines and the full cyclobenzaprine withdrawal guide.
Is cyclobenzaprine legal?
In the United States, cyclobenzaprine is a prescription-only medicine. It is not a federal controlled substance, so the only legal way to get it is with a prescription from a licensed clinician, filled by a licensed pharmacy.
- Refills: refills follow ordinary prescription rules.
- Pharmacies: a legitimate pharmacy always requires a valid prescription, has a licensed pharmacist available, and is licensed in the state it ships to. The FDA's BeSafeRx tool and the NABP's safe pharmacy list help confirm a pharmacy is real.
- Cost: generic cyclobenzaprine tablets are among the least expensive prescription medicines.
Outside the United States
| Country | Status |
|---|---|
| Canada | Prescription only |
| United Kingdom | Not licensed |
| International | Not under UN control |
Frequently asked questions about cyclobenzaprine
Is cyclobenzaprine a narcotic?
No. It is not an opioid or a controlled substance.
Will cyclobenzaprine make me sleepy?
Very likely. Drowsiness is its most common side effect, so many people take it at night.
Can you take cyclobenzaprine with ibuprofen?
Yes. They are often used together for back pain.
Can cyclobenzaprine be taken with antidepressants?
With caution. It can cause serotonin syndrome with SSRIs and SNRIs and must not be taken with MAO inhibitors.
How long can I take Flexeril?
It is approved for 2 to 3 weeks.
Sources
- Flexeril (cyclobenzaprine) prescription information, DailyMed, US National Library of Medicine.
- BeSafeRx: Your Source for Online Pharmacy Information, US Food and Drug Administration.
- Safe Pharmacy, National Association of Boards of Pharmacy.
- One Pill Can Kill, US Drug Enforcement Administration.
- International drug control conventions, United Nations Office on Drugs and Crime.
This guide summarizes the sourced DoseUses Cyclobenzaprine profile, written from FDA labels, DEA rules, and federal health agencies. It is educational and is not medical advice. Last updated October 5, 2026.
