Side Effects of Flexeril (cyclobenzaprine)

Flexeril (cyclobenzaprine) is a muscle relaxant used with rest and physical therapy for short-term relief of muscle spasms associated with acute painful muscle and skeletal conditions. It is only for short-term use, up to two or three weeks. 

Flexeril is used to relieve muscle spasm when the spasm is due to local problems, that is, in the muscle itself and not in the nerves controlling the muscles. Flexeril has no effect on muscle function. Flexeril seems to accomplish its beneficial effect through a complex mechanism within the nervous system, probably in the brainstem.

Common side effects of Flexeril include drowsiness, dry mouth, fatigue, headaches, and dizziness.

Other reported side effects of Flexeril include nausea, constipation, blurred vision, unpleasant taste, nervousness, confusion, acid reflux, and abdominal pain or discomfort.

Serious side effects of Flexeril include seizures, abnormal heartbeats, stroke, heart attacks, and heatstroke.

Abrupt cessation of Flexeril after prolonged therapy may cause withdrawal symptoms such as headaches, nausea, and weakness.

Drug interactions of Flexeril include tricyclic antidepressants because Flexeril is chemically related to this class of drugs.

Flexeril should not be taken with or within two weeks of any monoamine oxidase inhibitors (MAOs) for example, isocarboxazid, phenelzine, tranylcypromine, and procarbazine.

High fever, convulsions, and even death can occur when these drugs are used together.

Flexeril interacts with other medications and drugs that slow the brain's processes, such as alcohol, barbiturates, benzodiazepines, and narcotics. 

There are no adequate studies of Flexeril in pregnant women. Studies in animals suggest no important effects on the fetus. Flexeril therefore can be used in pregnancy if the physician feels it is necessary. 

It is unknown if Flexeril is secreted in breast milk. Since it is related to tricyclic antidepressants, some of which are excreted in breast milk, caution is advised in using Flexeril in women who are breastfeeding.

What are the common side effects of Flexeril?

The most common side effects of cyclobenzaprine include:

• Drowsiness
• Dry mouth
• Fatigue
• Headaches
• Dizziness

Other reported side effects include:

• Nausea
• Constipation
• Blurred vision,
• Unpleasant taste
• Nervousness
• Confusion
• Acid reflux
• Abdominal pain or discomfort

What are the serious side effects of Flexeril?

Possible serious side effects include:

• Seizures
• Abnormal heart beats
• Stroke
• Heart attacks
• Heat stroke

Abrupt cessation after prolonged therapy may cause withdrawal symptoms such as headaches, nausea, and weakness.

Drug Abuse And Dependence

• Pharmacologic similarities among the tricyclic drugs require that certain withdrawal symptoms be considered when Flexeril is administered, even though they have not been reported to occur with this drug.
• Abrupt cessation of treatment after prolonged administration rarely may produce nausea, headache, and malaise. These are not indicative of addiction.

• Flexeril may have life-threatening interactions with MAO inhibitors.
• Flexeril may enhance the effects of alcohol, barbiturates, and other CNS depressants.
• Tricyclic antidepressants may block the antihypertensive action of guanethidine and similarly acting compounds.
• Tricyclic antidepressants may enhance the seizure risk in patients taking tramadol.

Incidence of most common adverse reactions in the 2 double-blind‡, placebo-controlled 5 mg studies (incidence of > 3% on Flexeril 5 mg):

Adverse reactions which were reported in 1% to 3% of the patients were:

• abdominal pain,
• acid regurgitation,
• constipation,
• diarrhea,
• dizziness,
• nausea,
• irritability,
• mental acuity decreased,
• nervousness,
• upper respiratory infection, and
• pharyngitis.

The following list of adverse reactions is based on the experience in 473 patients treated with Flexeril 10 mg in additional controlled clinical studies, 7607 patients in the post-marketing surveillance program, and reports received since the drug was marketed. The overall incidence of adverse reactions among patients in the surveillance program was less than the incidence in the controlled clinical studies.

The adverse reactions reported most frequently with Flexeril were

• drowsiness,
• dry mouth and
• dizziness.

The incidence of these common adverse reactions was lower in the surveillance program than in the controlled clinical studies:

‡ Note: Flexeril 10 mg data are from one clinical trial. Flexeril 5 mg and placebo data are from two studies.

Among the less frequent adverse reactions, there was no appreciable difference in incidence in controlled clinical studies or in the surveillance program. Adverse reactions which were reported in 1% to 3% of the patients were:

• fatigue/tiredness,
• asthenia,
• nausea,
• constipation,
• dyspepsia,
• unpleasant taste,
• blurred vision,
• headache,
• nervousness, and
• confusion.

The following adverse reactions have been reported in post-marketing experience or with an incidence of less than 1% of patients in clinical trials with the 10 mg tablet:

• Body as a Whole: Syncope; malaise.
• Cardiovascular: Tachycardia; arrhythmia; vasodilatation; palpitation; hypotension.
• Digestive: Vomiting; anorexia; diarrhea; gastrointestinal pain; gastritis; thirst; flatulence; edema of the tongue; abnormal liver function and rare reports of hepatitis, jaundice and cholestasis.
• Hypersensitivity: Anaphylaxis; angioedema; pruritus; facial edema; urticaria; rash.
• Musculoskeletal: Local weakness.
• Nervous System and Psychiatric: Seizures, ataxia; vertigo; dysarthria; tremors; hypertonia; convulsions; muscle twitching; disorientation; insomnia; depressed mood; abnormal sensations; anxiety; agitation; psychosis, abnormal thinking and dreaming; hallucinations; excitement; paresthesia; diplopia.
• Skin: Sweating.
• Special Senses: Ageusia; tinnitus.
• Urogenital: Urinary frequency and/or retention.

Causal Relationship Unknown

Other reactions, reported rarely for Flexeril under circumstances where a causal relationship could not be established or reported for other tricyclic drugs, are listed to serve as alerting information to physicians:

• Body as a whole: Chest pain; edema.
• Cardiovascular: Hypertension; myocardial infarction; heart block; stroke.
• Digestive: Paralytic ileus, tongue discoloration; stomatitis; parotid swelling.
• Endocrine: Inappropriate ADH syndrome.
• Hematic and Lymphatic: Purpura; bone marrow depression; leukopenia; eosinophilia; thrombocytopenia.
• Metabolic, Nutritional and Immune: Elevation and lowering of blood sugar levels; weight gain or loss.
• Musculoskeletal: Myalgia.
• Nervous System and Psychiatric: Decreased or increased libido; abnormal gait; delusions; aggressive behavior; paranoia; peripheral neuropathy; Bell's palsy; alteration in EEG patterns; extrapyramidal symptoms.
• Respiratory: Dyspnea.
• Skin: Photosensitization; alopecia.
• Urogenital: Impaired urination; dilatation of urinary tract; impotence; testicular swelling; gynecomastia; breast enlargement; galactorrhea.