Does Flexeril (cyclobenzaprine) cause side effects?

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

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

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 important side effects of Flexeril (cyclobenzaprine)?

The most common side effects of cyclobenzaprine include:

Other reported side effects include:

Possible serious side effects include:

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

Flexeril (cyclobenzaprine) side effects list for healthcare professionals

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

  Flexeril 5 mg
N=464
Flexeril 10 mg
N=249
Placebo
N=469
Drowsiness 29% 38% 10%
Dry Mouth 21% 32% 7%
Fatigue 6% 6% 3%
Headache 5% 5% 8%

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

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

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.

  Clinical Studies With Flexeril 10 mg Surveillance Program With Flexeril 10 mg
Drowsiness 39% 16%
Dry Mouth 27% 7%
Dizziness 11% 3%

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:

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:

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:

Does Flexeril (cyclobenzaprine) cause addiction or withdrawal symptoms?

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.

What drugs interact with Flexeril (cyclobenzaprine)?

Ultram (tramadol HCl tablets, Ortho-McNeil Pharmaceutical)
Ultracet (tramadol HCl and acetaminophen tablets, Ortho-McNeil Pharmaceutical)

Summary

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. 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. There are no adequate studies of Flexeril in pregnant women. 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.

Treatment & Diagnosis

Medications & Supplements

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Medically Reviewed on 1/5/2021
References
FDA Prescribing Information

Professional side effects and drug interactions sections courtesy of the U.S. Food and Drug Administration.