Flexeril side effects and drug information

Flexeril is a brand name and a kind of medication that is used to aid in relaxing specific muscles in the body. This medicine is available as well in a generic form which is called cyclobenzaprine.  


ChemSpider ID

Protein binding93%
CAS ID303-53-7
Molar mass275.387 g/mol


This was formulated to help in relieving pain, stiffness, and other discomforts that are caused by strains, sprains, or injuries to the muscles. Though it offers relief for these types of muscle pains or strains, this medicine still is no match for rest, exercise, and physical therapy.

Flexeril or cyclobenzaprine works on the central nervous system to produce its muscle relaxant effects. However, though it functions on the central nervous system, it may also be the root or trigger for the side effects.

This muscle relaxant medication may be used as part of combination therapy.

How It Works

Flexeril comes from a class of drugs called muscle relaxants. These types of drugs work to relieve pain, stiffness, and other discomforts in certain muscles. Though, it is not precisely known how this medication functions in relaxing the muscle

Cyclobenzaprine may also tend to decrease the signals from the brain that instructs your muscles to cause spasm.

Side Effects

As in any other drugs, this muscle relaxant may also potentially cause some side effects to its users, which can either be some common side effects or serious ones depending on the body’s reaction and the person’s condition.

It is said that its oral tablet form may trigger dizziness and drowsiness, which may occur more likely after a few hours from taking it.

Some of the more common side effects, as well as its serious side effects, will be mentioned here.

Common Side Effects

Serious Side Effects

For the symptoms in relation to heart problems, the person may experience the following:

  • Fainting
  • Heart palpitations
  • Confusion
  • Trouble speaking or understanding
  • Loss of control or numbness in the face, arms or legs
  • Trouble seeing in 1 or both eyes

For the symptoms involving serotonin syndrome, the user may experience:

  • Agitation
  • Hallucinations
  • Seizures
  • Nausea

Indication and dosage

The following information on the average dose for this medication is for its oral tablet version. Medicine dosage, drug form, and how often you take a drug may vary according to different factors such as:

  • Patient’s age
  • The user’s condition that is being treated
  • How severe the patient’s condition is
  • If the individual has other medical conditions
  • How the person’s body reacts to the first dose

Forms and strengths of this medication in its generic and brand name versions:

The generic name is cyclobenzaprine, and it comes in an oral tablet form. It is available in 5mg, 7.5mg, and 10mg strengths

For the brand name version, the drug is called Fexmid, and it is also available in oral tablet form. It comes in 1 strength, which is 7.5mg.

The dosage for muscle spasm relief are the following:

  • For the adult dose, which is for ages 18 to 64 years old, this medicine with 5 to 10mg strength must be taken three times a day.
  • For the child dose, which is designed for 15 to 17 years old, they may take 5 to 10mg strength three times per day.
  • For children 0 to 14 years old, this medicine is not recommended for them as this should not be used for those who are younger than 15 years old.

For the seniors with ages from 65 years old and up, since their kidneys do not function properly as they used to, their bodies may process the drug more slowly which will then cause the drugs to stay in the body for a much longer time, and might lead to an increased risk of side effects. Because of this reason, the doctor may either lower their dosage or give a different dosing schedule so that the drugs will be prevented from building up too much in the body.

Special Dosage Considerations

The special and appropriate dosage is given for individuals who have issues in the liver. If the person’s liver problem is mild, the doctor may adjust and provide a lower dosage or may give a different dosing schedule. But if the liver problem is either moderate or severe, do not take this drug as it may cause further complications.

Drug Interaction

This medication may also have the potential to interact with other drugs, which includes vitamins and herbal medicines. To avoid drug interactions, it is better to consult with your doctor and inform you about all your other medications so that he or she can manage your medicines carefully.  

Interactions that increase your risk of side effects

There are some types of drugs that may give you an increased risk of experiencing side effects when you take cyclobenzaprine. The reason for this is because the amount of cyclobenzaprine is increased in your body.

The following drugs may cause an increased risk for side effects:

  • Benzodiazepines such as triazolam, alprazolam, and midazolam. Taking this type of drug along with cyclobenzaprine may lead to increased drowsiness and sedation
  • Barbiturates such as phenobarbital
  • Drugs used as a treatment for depression like fluoxetine, venlafaxine, amitriptyline, or bupropion. When you take any of these with cyclobenzaprine, you may be at a higher risk for serotonin syndrome.
  • Verapamil
  • Anticholinergic drugs such as tolterodine or oxybutynin. This may interact with cyclobenzaprine and give a higher risk for side effects like dry mouth and unable to urinate.

Interactions that can make your drug less effective

Cyclobenzaprine may also change how certain drugs function, which may result in medicine not working correctly. An example of this is guanethidine. When this is used along with cyclobenzaprine, its blood pressure- lowering effects may be hindered and may lead to an increase in blood pressure.

Warnings and Precautions

It is always important to check product labels before usage to know the appropriate dosage and the possible side effects of the medicine. Or to ensure safety, consult with your doctor first and inform him or her if you are allergic to the medication, or if you have any other allergies as the drug may have some inactive components and may cause allergic reactions.

It is also best to inform your healthcare provider about your medical history, most especially when you have these types of conditions: liver disease, overactive thyroid, heart problems, difficulty urinating, or glaucoma.

This medicine may cause drowsiness or dizziness as well, so if you are taking this product, avoid driving or operating machinery.  

The use of cyclobenzaprine is also not advisable to be taken along with alcohol or cannabis, as it may make you feel dizzier.

Prior to surgery, it is best to also inform your doctor or dentist about all your other medicines, including prescription, nonprescription, herbal, etc. 

Older adults may have more sensitivity to experiencing side effects from this medicine, such as drowsiness, confusion, constipation, or trouble urinating. Drowsiness can be dangerous for older people as it may increase their risk of falling.

Important Warnings

Some more important warnings that people using this product should know about:

Serotonin syndrome warning

This drug may result in a life-threatening condition known as serotonin syndrome. This occurs when medicines cause too much serotonin to build up in the body. Users of this drug should watch out for these symptoms of agitation, hallucinations, seizures, or nausea. The risk becomes higher if you take cyclobenzaprine along with other medicines such as antidepressants.

Effects on the heart warning

This type of medication may cause heart arrhythmias such as heart rate or rhythm problems, and the risk becomes greater if you have heart problems or if you are taking anti-depression drugs with it. If heart problems are not treated, it may lead to a heart attack or stroke.

Central nervous system warning

The use of cyclobenzaprine may cause drowsiness, hallucinations, dizziness or delusions, so if you are under this treatment, avoid driving or using machinery. The greater risk is for those who are 65 years old and up.

Overdose and Contraindications

It is advised that cyclobenzaprine should not be taken along with monoamine oxidase inhibitors or MAOIs such as selegiline, rasagiline, and tranylcypromine as it may lead to some harmful effects to the body. 

Also important to note is that taking cyclobenzaprine with MAOI or within 14 days of stopping an MAOI may cause an increased risk of some serious side effects like seizures.

Clinical Pharmacology

Cyclobenzaprine is a prescription drug that is also available in its brand name version called Fexmid. Though it also comes in a generic brand, the efficacy or the strength may not be the same as the one with the brand name even if it is less expensive. This drug is also available as an oral extended-release capsule.

Medication Guide


  • This medication may be consumed with or without food
  • It can be taken about the same time each day
  • You are allowed to cut or crush the tablet

Since not all pharmacies stock this product, call ahead when refilling your prescription.


  • This product must be stored at 25°C 
  • Should be kept away from light
  • Do not store in moist or damp areas

This prescription is refillable, but you do not need a new prescription for every refill as the doctor will write the number of refills authorized for this.