How the Skeletal Muscle Relaxer Flexeril Works

Muscle spasm treatment can range from massage and stretching on the holistic side to drugs on the conventional medical side.  To help release very tight muscle spasms, some people take a medication called Flexeril. Approved by the FDA in 1977, this drug is just one type of skeletal muscle relaxers on the market.

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

Woman holding a painful lower back
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Flexeril is the brand name for cyclobenzaprine. Cyclobenzaprine is the active ingredient and the generic form of the drug. Either way, this medication is available by prescription only.


Flexeril is sometimes used short-term, in conjunction with physical therapy and rest for acute cases of muscle injury, for example, sprains and strains. By relaxing ultra-tight muscles, it may help you make faster progress with your home exercise program.

As a skeletal muscle relaxer, cyclobenzaprine works in the central nervous system, indirectly influencing motor neurons of the brainstem and spinal cord to relieve spasms.

Administration and Dosage

As with any medication, Flexeril or cyclobenzaprine should be taken according to your healthcare provider's or pharmacist's instructions. It's meant to be used for about 2 to 3 weeks, i.e. not for the long term.

Generally, Flexeril or cyclobenzaprine is taken three times per day in tablet form. Immediate-release tablets contain 5 or 10 mg, while extended-release tablets contain 15 or 30 mg. Don't take it upon yourself to increase your dosage. If you feel you need more, speak to your healthcare provider.

If you miss a dose and it is now closer to the time of the next dose, resume taking the medication at the next scheduled time. Do not double dose to make up for a missed dosage.


Store cyclobenzaprine/Flexeril at room temperature or as instructed by the pharmacist.

Side Effects, Complications, and Precautions

While a number of potential side effects and complications for taking Flexeril exist, one of the most important is the potential for drowsiness or dizziness.

Don't drive or operate machinery or do anything that requires you to be alert while you're under the influence of Flexeril. Avoid alcohol, as well.

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

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

Serious side effects are rare, but if these do occur they require prompt medical attention. These include, but are not limited to:

  • Mental and mood changes, for example, hallucinations and confusion
  • Difficulty in urinating
  • Dark urine
  • Fast, pounding and/or irregular heartbeat
  • Fainting
  • Yellowing of the eyes and/or skin
  • Stomach and/or abdominal pain
  • Persistent nausea, vomiting and/or lack of appetite
  • Seizures
  • Loss of coordination

Note that the elderly will be more sensitive to side effects associated with this medication, and pregnant women should use Flexeril/cyclobenzaprine only if directed by their healthcare provider. The drug may also pass into breast milk.

Cyclobenzaprine may contain inactive ingredients that sometimes cause allergic reactions. Be sure to talk with your healthcare provider and pharmacist about this before taking this drug. You also should be transparent about any liver disease, thyroid problems, heart problems, glaucoma and/or difficulty urinating you've had in the past. This way, your healthcare provider can determine if you are at risk for a complication. The same is true for other medications you may be taking.

Frequently Asked Questions

  • What is cyclobenzaprine?

    Cyclobenzaprine is the active ingredient in Flexeril, a prescription drug that helps relieve muscle spasms. These muscle spasms or cramps can result from 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, and more. Although serious side effects are rare, they can include hallucinations or confusion, difficulty urinating, irregular heartbeat, fainting, seizures, and loss of coordination. Medical attention is recommended if these serious side effects occur.

  • What is the typical dosage of Flexeril?

    Flexeril or cyclobenzaprine is available in immediate-release tablets of 5 or 10 mg as well as extended-release tablets in 15 or 30 mg. In most cases, it is taken three times daily for about two to three weeks. A healthcare provider will prescribe a specific dosage and schedule for your needs.

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  1. U.S. Food and Drug Administration. FLEXERIL® (CYCLOBENZAPRINE HCL) TABLETS.