Flexeril is a muscle relaxant. It works by blocking nerve impulses (or pain sensations) that are sent to your brain.

Flexeril is used together with rest and physical therapy to treat skeletal muscle conditions such as pain, injury, or spasms.

Flexeril may also be used for purposes not listed in this medication guide.

You should not use Flexeril if you have a thyroid disorder, heart block, congestive heart failure, a heart rhythm disorder, or you have recently had a heart attack.

Do not use cyclobenzaprine if you have taken an MAO inhibitor in the past 14 days, such as isocarboxazid, linezolid, phenelzine, rasagiline, selegiline, or tranylcypromine.

Flexeril is usually taken once daily for only 2 or 3 weeks. Follow all directions on your prescription label and read all medication guides or instruction sheets. Your doctor may occasionally change your dose.

Take the medicine at the same time each day.

Call your doctor if your symptoms do not improve after 3 weeks, or if they get worse.

Store at room temperature away from moisture, heat, and light.

You should not use Flexeril if you are allergic to cyclobenzaprine, or if you have:

• a thyroid disorder;

• heart block, heart rhythm disorder, congestive heart failure; or

• if you have recently had a heart attack.

Flexeril is not approved for use by anyone younger than 15 years old.

Do not use Flexeril if you have taken an MAO inhibitor in the past 14 days. A dangerous drug interaction could occur. MAO inhibitors include isocarboxazid, linezolid, phenelzine, rasagiline, selegiline, and tranylcypromine.

Some medicines can interact with cyclobenzaprine and cause a serious condition called serotonin syndrome. Be sure your doctor knows if you also take stimulant medicine, opioid medicine, herbal products, or medicine for depression, mental illness, Parkinson's disease, migraine headaches, serious infections, or prevention of nausea and vomiting. Ask your doctor before making any changes in how or when you take your medications.

To make sure Flexeril is safe for you, tell your doctor if you have ever had:

• liver disease;

• glaucoma;

• enlarged prostate; or

• problems with urination.

It is not known whether cyclobenzaprine will harm an unborn baby. Tell your doctor if you are pregnant or plan to become pregnant.

It may not be safe to breast-feed while using this medicine. Ask your doctor about any risk.

Older adults may be more sensitive to the effects of this medicine.

Take the medicine as soon as you can, but skip the missed dose if it is almost time for your next dose. Do not take two doses at one time.

Seek emergency medical attention or call the Poison Help line at 1-800-222-1222. An overdose of cyclobenzaprine can be fatal.

Overdose symptoms may include severe drowsiness, vomiting, fast heartbeats, tremors, agitation, or hallucinations.

Avoid driving or hazardous activity until you know how cyclobenzaprine will affect you. Your reactions could be impaired.

Avoid drinking alcohol. Dangerous side effects could occur.

Get emergency medical help if you have signs of an allergic reaction to Flexeril: hives; difficult breathing; swelling of your face, lips, tongue, or throat.

Stop using Flexeril and call your doctor at once if you have:

• fast or irregular heartbeats;

• chest pain or pressure, pain spreading to your jaw or shoulder; or

• sudden numbness or weakness (especially on one side of the body), slurred speech, balance problems.

Seek medical attention right away if you have symptoms of serotonin syndrome, such as: agitation, hallucinations, fever, sweating, shivering, fast heart rate, muscle stiffness, twitching, loss of coordination, nausea, vomiting, or diarrhea.

Serious side effects may be more likely in older adults.

Common Flexeril side effects may include:

• drowsiness, tiredness;

• headache, dizziness;

• dry mouth; or

• upset stomach, nausea, constipation.

This is not a complete list of side effects and others may occur. Call your doctor for medical advice about side effects. You may report side effects to FDA at 1-800-FDA-1088.

Using Flexeril with other drugs that make you drowsy can worsen this effect. Ask your doctor before using opioid medication, a sleeping pill, a muscle relaxer, or medicine for anxiety or seizures.

Tell your doctor about all your other medicines, especially:

• bupropion (Zyban, for smoking cessation);

• meperidine;

• tramadol;

• verapamil;

• narcotics;

• cold or allergy medicine that contains an antihistamine (Benadryl and others);

• medicine to treat Parkinson's disease;

• medicine to treat excess stomach acid, stomach ulcer, motion sickness, or irritable bowel syndrome;

• medicine to treat overactive bladder; or

• bronchodilator asthma medication.

This list is not complete. Other drugs may interact with cyclobenzaprine, including prescription and over-the-counter medicines, vitamins, and herbal products. Not all possible drug interactions are listed here.