How the skeletal muscle relaxant Flexeril (cyclobenzaprine) works

Muscle spasm treatments range from holistic aspects of massage and stretching to traditional medical aspects of medication. To help relieve very tense muscle spasms, some people take a drug called cyclobenzaprine. Approved by the US Food and Drug Administration (FDA) in 1977 under the name Flexeril, it is only one skeletal muscle relaxant on the market.

Below is a fact sheet about cyclobenzaprine, including side effects, general information, dosage and storage information, and more.


Flexeril is the original brand name for cyclobenzaprine. It is available now as a generic drug, and an expanded version is available as a generic drug or under the Amrix brand name.


Cyclobenzaprine is used short-term, along with physical therapy and rest, in cases of acute muscle injury, such as sprains and strains. By relaxing super tight muscles, it can help you make faster progress in your home exercise program.

Instead of acting directly on the muscles, cyclobenzaprine acts on the central nervous system—especially the brainstem—to relieve spasms.

Administration and Dosing

As with any medicine, cyclobenzaprine should be taken as directed by your healthcare provider or pharmacist. It is intended to be used for about 2 to 3 weeks, not long term.

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Typically, cyclobenzaprine is taken in tablet form 3 times a day. Immediate-release tablets contain 5 or 10 mg, while extended-release tablets contain 15 or 30 mg. Do not increase the dose yourself. If you feel you need more, talk to your healthcare provider.

If you miss a dose and it’s now closer to your next dose, resume it at the next scheduled time. Do not double your dose to make up for a missed dose.


Store cyclobenzaprine at room temperature or as directed by your pharmacist.

Side Effects, Complications, and Precautions

Among the potential side effects and complications of taking cyclobenzaprine, one of the most important is possible drowsiness or dizziness.

After taking cyclobenzaprine, do not drive or operate machinery or do anything that requires you to be alert. Also, avoid alcohol and other sedatives, which can exacerbate the risk.

Some of the most common side effects include, but are not limited to:

  • dry mouth
  • drowsiness
  • Dizziness
  • tired
  • constipate
  • blurred vision
  • unpleasant taste
  • nervous
  • Puzzled
  • abdominal pain or discomfort
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Serious side effects are rare, but if they do occur, prompt medical attention is required. These include but are not limited to:

  • mental and emotional changes, such as hallucinations and confusion
  • difficulty urinating
  • dark urine
  • fast, pounding, and/or irregular heartbeat
  • syncope
  • yellowing of the eyes and/or skin
  • stomach and/or abdominal pain
  • persistent nausea, vomiting and/or loss of appetite
  • Seizures
  • loss of coordination

Note that older adults are more sensitive to the side effects associated with this drug, and pregnant women should only use cyclobenzaprine under the direction of their healthcare provider. This drug may also pass into breast milk.

Cyclobenzaprine may contain inactive ingredients that can sometimes cause allergic reactions. Be sure to discuss this with your healthcare provider and pharmacist before taking this medication. You should also be transparent about any past liver disease, thyroid problems, heart problems, glaucoma, and/or difficulty urinating. This way, your healthcare provider can determine if you are at risk for complications. The same goes for other medicines you may be taking.

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Frequently Asked Questions

  • What is cyclobenzaprine?

    Cyclobenzaprine is a prescription drug that helps relieve muscle spasms. These muscle cramps or cramps can be caused by dehydration, muscle fatigue, electrolyte depletion, pinched nerves, and other conditions.

  • What are the side effects of cyclobenzaprine?

    The biggest side effect of cyclobenzaprine is possible drowsiness or dizziness. Other side effects include dry mouth, constipation, nervousness, abdominal pain, etc. Although serious side effects are rare, they can include hallucinations or confusion, trouble urinating, irregular heartbeat, fainting, seizures, and loss of coordination. If these serious side effects occur, medical attention is advised.

  • What is the typical dose of cyclobenzaprine?

    Cyclobenzaprine is available in immediate-release tablets of 5 or 10 mg and extended-release tablets of 15 or 30 mg. In most cases, it is taken 3 times a day for about two to three weeks. A healthcare provider will prescribe a specific dose and schedule based on your needs.