Flexeril (Cyclobenzaprine): Uses, Side Effects, Dosages

A muscle relaxer commonly used to treat spasms

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Flexeril (cyclobenzaprine) is a muscle relaxer prescribed for very tight muscle spasms, as well as pain and discomfort from muscle injuries. It is generally used short term along with physical therapy and rest for acute injuries, such as tendon or ligament sprains and muscle strains. 

Flexeril is actually the brand-name version of the drug, and it is discontinued. However, the name is still often used interchangeably with cyclobenzaprine (the generic name). An extended-release version is available both as a generic or under the brand name Amrix.

In this article, you'll learn all about Flexeril, how it works, the proper Flexeril dosage, side effects and precautions, and what drugs shouldn't be combined with cyclobenzaprine.

Woman holding a painful lower back
Tom Merton / OJO Images / Getty Images

How Flexeril Works

Flexeril doesn't work directly on the muscles. Instead, it works in the central nervous system—particularly the brain stem—to relieve spasms.

By relaxing ultra-tight muscles, Flexeril may help you make faster progress with your home exercise program.

Flexeril Dosage

As with any medication, you should stick to the Flexeril dosage prescribed by your healthcare provider. This drug is meant to be used for between two and three weeks at a time.

Generally, Flexeril directions stay to take three tablets a day.

  • Immediate-release tablets contain either 5 milligrams (mg) or 10 mg
  • Extended-release tablets contain either 15 mg or 30 mg

Flexeril dosages for off-label (non-approved) uses such as fibromyalgia and sciatica are not established. Fibromyalgia studies have used doses between 1 mg and 4 mg before bed.

If you think you need more of this drug, talk to your healthcare provider. Don't increase the dosage on your own.

Missed a Flexeril Dose?

If you miss a dose and remember closer to the time of the next dose, skip the forgotten dose and take your next one at its scheduled time. Do not double the dose to make up for a missed pill.

Flexeril Side Effects and Risks

Cyclobenzaprine carries a risk of side effects and may not be safe for some people. It can also interact negatively with certain drugs.

Side Effects

Some common side effects of Flexeril include:

  • Dry mouth
  • Drowsiness
  • Dizziness
  • Tiredness
  • Constipation
  • Blurred vision
  • Unpleasant taste
  • Nervousness
  • Confusion
  • Abdominal pain or discomfort

Drowsiness is so common that Flexeril is sometimes prescribed for improving sleep, especially in people with chronic pain disorders such as fibromyalgia.


Don’t drive or operate machinery or do anything that requires you to be alert after taking cyclobenzaprine. Also, avoid alcohol and other depressants, which can compound the risks.

Serious side effects are rare, but if they do occur they require prompt medical attention. These include:

  • Mental and mood changes (e.g., hallucinations, confusion)
  • Difficulty urinating
  • Dark urine
  • Fast, pounding, and/or irregular heartbeat
  • Fainting
  • Yellowing of the eyes and/or skin (jaundice)
  • Stomach and/or abdominal pain
  • Persistent nausea, vomiting, and/or lack of appetite
  • Seizures
  • Loss of coordination

Older people are more sensitive to Flexeril side effects.

Allergies and Precautions

Flexeril contains inactive ingredients that sometimes cause allergic reactions. Especially if you have known drug allergies, be sure to talk to your healthcare provider and pharmacist about this before taking this drug.

To avoid possible complications of Flexeril, let your healthcare provider know if you have:

  • Liver disease
  • Thyroid problems
  • Depression, bipolar disorder, or other mental health conditions
  • A seizure disorder
  • Heart problems
  • Glaucoma
  • Difficulty urinating now or in the past

Safety in children under 15 hasn't been established.

Pregnancy and Breastfeeding

If you're pregnant or breastfeeding, you should only use Flexeril if it's approved by your OB/GYN. The drug may pass into breast milk.

What to Avoid While Taking Flexeril

Some medications shouldn't be taken together due to negative interactions. Flexeril shouldn't be taken with:

  • Opioids: Vicodin (hydrocodone/acetaminophen), codeine, OxyContin (oxycodone)
  • Tricyclic antidepressants: Amitriptyline, Norpramin (desipramine), Pamelor (nortriptyline)
  • Serotonin norepinephrine reuptake inhibitor (SNRI) antidepressants: Cymbalta (duloxetine), Savella (milnacipran)
  • Other antidepressants: Wellbutrin (bupropion), Ultram (tramadol)
  • MAO Inhibitors (MAOIs): Marplan (isocarboxazid), Nardil (phenelzine), Emsam (selegiline)
  • Anticholinergics: Atropen (atropine), Cogentin (beztropine mesylate), Enablex (darifenacin), Lotronex (alosetron)
  • Voltage gated potassium channel blockers: Firdapse/Ruzurgi (amifampridine), Ampyra (dalfampridine)
  • Anti-anxiety drugs: Xanax (alprazolam), Klonopin (clonazepam), Valium (diazepam)
  • Sedatives (sleeping pills): Ambien (zolpidem), Lunesta (eszopiclone), Luminal (phenobarbital), Xyrem (sodium oxybate)
  • Sedating antihistamines/cough suppressants: Benadryl (diphenhydramine), Solotuss (carbetapentane), Chlor-Trimeton (chlorpheniramine), Zyrtec (cetirizine)
  • COMT inhibitors for Parkinson's disease: Atmet/Duopa (carbidopa/levodopa), Comtan (entacapone), Tasmar (tolcapone)
  • Anti-seizure drugs: Lyrica (pregabalin), Neurontin (gabapentin)

Fibromyalgia Drug Warning

While Flexeril is commonly prescribed for fibromyalgia, it can interact negatively with all FDA-approved drugs for this condition (Lyrica, Cymbalta, Savella) and many that are prescribed off-label for it (Neurontin, tricyclic antidepressants, sedatives, and opioids). Be sure your healthcare provider and pharmacist know about everything you're taking.


Flexeril (cyclobenzaprine) is a muscle relaxer that works on the central nervous system. It comes in multiple dosages plus regular and extended-release forms.

Watch for side effects such as drowsiness, dizziness, and confusion. Severe side effects such as hallucinations, urine changes, jaundice, and seizures need immediate medical attention.

Don't take this drug if you're pregnant, breastfeeding, or under age 15. Tell you healthcare provider about all your medical conditions and other medications.

Frequently Asked Questions

  • What drug class is Flexeril a part of?

    Flexeril is a central nervous system muscle relaxant. Drugs in this class are not considered controlled substances.

  • Are Flexeril and Robaxin the same thing?

    They both relax muscles, but are different drugs. Flexeril contains cyclobenzaprine, while Robaxin contains methocarbamol.

6 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. MedlinePlus. Cyclobenzaprine.

  2. National Institutes of Health, U.S. National Library of Medicine: DailyMed. Label: Cyclobenzaprine hydrochloride - cyclobenzaprine hydrochloride tablet, film coated.

  3. National Institutes of Health, U.S. National Library of Medicine: DailyMed. Drug label: Amrix- cyclobenzaprine hydrochloride capsule, extended release.

  4. Prescribers' Digital Reference. Cyclobenzaprine hydrochloride - drug summary.

  5. National Institutes of Health, U.S. National Library of Medicine: ClinicalTrials.gov. Pilot study of cyclobenzaprine for the treatment of sleep distrubance in aromatase inhibitor-treated breast cancer patients.

  6. Drug Enforcement Administration. Cyclobenzaprine (Brand Name: Flexeril, Amrix).

By Anne Asher, CPT
Anne Asher, ACE-certified personal trainer, health coach, and orthopedic exercise specialist, is a back and neck pain expert.