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Flexeril Side Effects

Generic Name: cyclobenzaprine

Note: This page contains information about the side effects of cyclobenzaprine. Some of the dosage forms included on this document may not apply to the brand name Flexeril.

In Summary

Common side effects of Flexeril include: drowsiness and xerostomia. Other side effects include: fatigue. See below for a comprehensive list of adverse effects.

For the Consumer

Applies to cyclobenzaprine: oral capsule extended release, oral suspension, oral tablet

In addition to its needed effects, some unwanted effects may be caused by cyclobenzaprine (the active ingredient contained in Flexeril). In the event that any of these side effects do occur, they may require medical attention.

Stop taking cyclobenzaprine and get emergency help immediately if any of the following effects occur:

  • Changes in the skin color of the face
  • fast or irregular breathing
  • large swellings that look like hives on the face, eyelids, mouth, lips, and/or tongue
  • puffiness or swelling of the eyelids or the area around the eyes
  • shortness of breath, troubled breathing, tightness in chest, and/or wheezing
  • skin rash, hives, or itching

Severity: Major

You should check with your doctor immediately if any of these side effects occur when taking cyclobenzaprine:

  • Fainting
Symptoms of overdose:
  • Convulsions (seizures)
  • drowsiness (severe)
  • dry, hot, flushed skin
  • fast or irregular heartbeat
  • hallucinations (seeing, hearing, or feeling things that are not there)
  • increase or decrease in body temperature
  • troubled breathing
  • unexplained muscle stiffness
  • unusual nervousness or restlessness (severe)
  • vomiting (occurring together with other symptoms of overdose)

Severity: Moderate

If any of the following side effects occur while taking cyclobenzaprine, check with your doctor or nurse as soon as possible:

  • Clumsiness or unsteadiness
  • confusion
  • mental depression or other mood or mental changes
  • problems in urinating
  • ringing or buzzing in the ears
  • skin rash, hives, or itching occurring without other symptoms of an allergic reaction listed above
  • unusual thoughts or dreams
  • yellow eyes or skin

Severity: Minor

Some of the side effects that can occur with cyclobenzaprine may not need medical attention. As your body adjusts to the medicine during treatment these side effects may go away. Your health care professional may also be able to tell you about ways to reduce or prevent some of these side effects. If any of the following side effects continue, are bothersome or if you have any questions about them, check with your health care professional:

More common:
  • Blurred vision
  • dizziness or lightheadedness
  • drowsiness
  • dryness of mouth
Less common or rare:
  • Bloated feeling or gas, indigestion, nausea or vomiting, or stomach cramps or pain
  • constipation
  • diarrhea
  • excitement or nervousness
  • frequent urination
  • general feeling of discomfort or illness
  • headache
  • muscle twitching
  • numbness, tingling, pain, or weakness in hands or feet
  • pounding heartbeat
  • problems in speaking
  • trembling
  • trouble in sleeping
  • unpleasant taste or other taste changes
  • unusual muscle weakness
  • unusual tiredness

For Healthcare Professionals

Applies to cyclobenzaprine: compounding powder, oral capsule extended release, oral tablet, oral and topical kit

Nervous system

Elderly patients may be particularly susceptible to the sedation and confusion which may accompany cyclobenzaprine (the active ingredient contained in Flexeril) therapy.[Ref]

Nervous system side effects are common and include drowsiness in up to 40% of treated patients and dizziness in up to 11%. Fatigue, asthenia, headache, nervousness, blurred vision, and confusion have also been reported.[Ref]


Other side effects have included anticholinergic effects such as dry mouth (in as many as 27% of treated patients), and, less frequently, dry skin, blurry vision, constipation, and an organic brain syndrome. Paradoxical sweating has also been reported.[Ref]


Psychiatric side effects including mania and psychosis have been reported rarely. A case of hallucinations has also been reported.[Ref]


Hypersensitivity side effects have included rash, angioedema, and pruritus. Anaphylaxis has been reported rarely.[Ref]


Cardiovascular side effects have included cardiac arrhythmias, tachycardia, and hypotension.[Ref]


1. "Product Information. Flexeril (cyclobenzaprine)." Merck & Co, Inc, West Point, PA.

2. Linden CH, Mitchiner JC, Lindzon RD, Rumack BH "Cyclobenzaprine overdosage." J Toxicol Clin Toxicol 20 (1983): 281-8

3. Engel PA, Chapron D "Cyclobenzaprine-induced delirium in two octogenarians." J Clin Psychiatry 54 (1993): 39

4. Heckerling PS, Bartow TJ "Paradoxical diaphoresis in cyclobenzaprine poisoning." Ann Intern Med 101 (1984): 881

5. Harsch HH "Mania in two patients following cyclobenzaprine." Psychosomatics 25 (1984): 791-3

6. Douglass MA, Levine DP "Hallucinations in an elderly patient taking recommended doses of cyclobenzaprine." Arch Intern Med 160 (2000): 1373

7. O'Riordan W, Gillette P, Calderon J, Stennes RL "Overdose of cyclobenzaprine, the tricyclic muscle relaxant." Ann Emerg Med 15 (1986): 592-3

Not all side effects for Flexeril may be reported. You should always consult a doctor or healthcare professional for medical advice. Side effects can be reported to the FDA here.

Disclaimer: Every effort has been made to ensure that the information provided is accurate, up-to-date and complete, but no guarantee is made to that effect. In addition, the drug information contained herein may be time sensitive and should not be utilized as a reference resource beyond the date hereof. This material does not endorse drugs, diagnose patients, or recommend therapy. This information is a reference resource designed as supplement to, and not a substitute for, the expertise, skill , knowledge, and judgement of healthcare practitioners in patient care. The absence of a warning for a given drug or combination thereof in no way should be construed to indicate safety, effectiveness, or appropriateness for any given patient. does not assume any responsibility for any aspect of healthcare administered with the aid of materials provided. The information contained herein is not intended to cover all possible uses, directions, precautions, warnings, drug interactions, allergic reactions, or adverse effects. If you have questions about the substances you are taking, check with your doctor, nurse, or pharmacist.