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.

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.

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.