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Side Effects of Flexeril

By Dr. Mary D. Daley ; Updated September 02, 2018

If you’re experiencing muscle spasms that are causing pain, your doctor may prescribe a short course of Flexeril. This medication is generally effective and safe, but it has a number of possible side effects you should be aware of.

What Is Flexeril?

Flexeril is the brand name for cyclobenzaprine, a common muscle relaxant. It’s used to reduce muscle spasms that often occur with muscle, ligament or bone injuries. By alleviating these spasms, Flexeril reduces pain and stiffness. It’s meant to be used in conjunction with rest and physical therapy.

Flexeril doesn’t act directly on muscles, so it usually doesn’t interfere with normal muscle function or strength. It acts primarily in a region of the brain to reduce the likelihood of muscle spasms.

The Flexeril molecule has the same general structure as tricyclic antidepressant medications, such as amitriptyline (Elavil) and imipramine (Tofranil). Because of this, Flexeril has many of the same side effects as these antidepressants.

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Read more: 10 Things to Know in Case of a Health Emergency

Minor Flexeril Side Effects

A number of relatively minor side effects may occur when using Flexeril, including:

  • Drowsiness
  • Dry mouth
  • Dizziness
  • Blurred vision
  • Unpleasant taste in the mouth
  • Loss of appetite
  • Nausea, abdominal pain or indigestion
  • Diarrhea or constipation
  • Nervousness or irritability
  • Confusion
  • Decreased ability to focus mentally

Drowsiness and dry mouth are the most common side effects, occurring in up to about one-third of people, according to the Flexeril prescribing information approved by the Food and Drug Administration (FDA). Dizziness is the third most common, occurring in about 10 percent of people.

Contact your physician if minor side effects become troublesome. Reducing the dose or frequency of Flexeril will often improve these symptoms. Taking Flexeril only at bedtime can reduce sleepiness in the daytime. Avoid driving a car, operating heavy machinery or making important decisions while taking Flexeril, as your ability to think clearly may be impaired.

Serious Flexeril Side Effects

Less commonly, Flexeril can cause serious side effects. If you notice any of these symptoms, stop taking this medication and seek prompt medical attention:

  • Fast, pounding or irregular heartbeat
  • Chest pain
  • Confusion, with weakness or lack of coordination
  • Severe drowsiness
  • Severe nausea
  • Severe abdominal pain
  • Yellowish skin or eyes
  • Seizures

Flexeril may also cause a potentially life-threatening condition called serotonin syndrome. According to the Mayo Clinic, symptoms of this condition include:

  • A high temperature
  • Profuse sweating
  • Rapid or irregular heartbeat
  • High blood pressure
  • Diarrhea
  • Severe confusion
  • Muscle rigidity or twitches
  • Shivering
  • Headache
  • Seizures
  • Fainting 
  • Coma

The syndrome is rare when Flexeril is used alone, but it’s more likely if Flexeril is taken with certain other drugs, especially monoamine oxidase (MAO) inhibitors.

Read more: How to Know When to Go to the ER vs. Urgent Care

Allergic Reactions and Overdose

As with any medication, some people may be allergic to Flexeril. Rarely, a serious allergic reaction may occur, producing:

  • Hives
  • A red face or body
  • Swelling of the face, mouth or throat
  • Shortness of breath
  • Wheezing
  • Dizziness
  • Fainting

Symptoms of an overdose may include:

  • Severe drowsiness
  • Fast heartbeat
  • High or low blood pressure
  • Vomiting
  • Hallucinations
  • Chest pain
  • Seizures
  • Slurred speech
  • Fever
  • Sweating
  • When severe, coma or death may occur. 

Call 911 or go to the emergency room immediately if you have symptoms of a severe allergic reaction or overdose.

Drug Interactions

Flexeril can interact with a number of medications. Before starting Flexeril, make sure your doctor is aware of all medications you are taking.

One of the most important interactions is between Flexeril and MAO inhibitors, such as isocarboxazid (Marplan) and phenelzine (Nardil). Taking MAO inhibitors with Flexeril significantly increases your likelihood of serotonin syndrome.

Drugs that cause sleepiness are another source of drug interaction, as they can increase the drowsiness produced by Flexeril. These include anti-anxiety drugs, sleeping medications, barbiturates, opioid painkillers, tranquilizers or psychiatric medications.

As noted at Drugs.com, many other drugs may also interact with Flexeril, including:

  • Medications used to speed up the heartbeat, such as atropine and glycopyrrolate
  • Some antinauseant medications: diphenhydrinate (Dramamine) and scopolamine (Transderm-Scop)
  • Benztropine (Cogentin), a medication for Parkinson’s disease
  • Some breathing medications: ipratroprium (Atrovent) and tiotropium (Spiriva)
  • Tramadol (Ultram), a pain-relieving medication 
  • Several medications for bladder conditions, such as oxybutynin (Ditropan, Oxytrol) and solifenacin (Vesicare)
  • Various medications for irritable bowel syndrome, such as dicyclomine (Bentyl) and hyoscyamine (Anaspaz, Cytosplaz)

Contraindications

Because of the possibility of a life-threatening drug interaction — serotonin syndrome — the FDA-approved prescribing information indicates that people who have taken any MAO inhibitor in the past 14 days should not take Flexeril. Two weeks are necessary to be certain that no more MAO inhibitor remains in your system.

Because Flexeril may cause a fast or other abnormal heart rhythm, the FDA indicates that it should not be used in people for whom this could be especially dangerous. This includes people with a recent heart attack, heart failure, a preexisting abnormal heart rhythm or hyperthyroidism (an overactive thyroid).

Because Flexeril is broken down by the liver, reduced liver function can lead to dangerously high Flexeril levels in the body. The FDA recommends taking a lower-than-normal dose if liver function is mildly impaired and completely avoiding Flexeril if liver function is moderately to severely reduced.

Additional Warnings

The FDA-approved prescribing information notes that Flexeril should be used for only a short time — for a maximum of two to three weeks. This is because there’s no good scientific evidence that it is effective when taken beyond this time.

As well, Flexeril is only approved for use in acute muscle, ligament or bone injuries ,and these conditions should not require a muscle relaxant after three weeks.

Flexeril is considered a pregnancy Class B drug. This means that it’s most likely safe to take when pregnant, but not enough research has been done to prove that it’s definitely not harmful to the baby. Before taking Flexeril, notify your doctor if you are or could be pregnant.

It is unknown whether Flexeril passes into the breast milk, but similar drugs — like the tricyclic antidepressants — do enter it. If you are breastfeeding, ask your pediatrician before taking Flexeril.

Flexeril can make some conditions worse. These include urinary retention (inability to adequately empty the bladder) and glaucoma (raised pressure within the eye). If you have either of these conditions, your doctor may recommend avoiding Flexeril.

The combination of alcohol and Flexeril can produce significant drowsiness. Do not drink alcohol while taking this medication.

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