Generic Name: cyclobenzaprine (sye kloe BEN za preen)
Brand Names: Amrix, Fexmid, Flexeril
Cyclobenzaprine is a muscle relaxant medication used to relieve skeletal muscle spasms and associated pain in acute musculoskeletal conditions. It is the most well-studied drug for this application, and it also has been used off-label for fibromyalgia treatment.
The mechanism of action for cyclobenzaprine is unclear. Studies from the 1980s in rats indicate that cyclobenzaprine activates the locus ceruleus in the brain stem, leading to an increased release of norepinephrine in the ventral horn of the spinal cord and the subsequent inhibitory action of norepinephrine on alpha motor neurons.
Cyclobenzaprine has been considered structurally related to the first-generation tricyclic antidepressants. Such tricyclics, including amitriptyline, act to inhibit the uptake of norepinephrine, resulting in increased transynaptic norepinephrine concentration. They have been shown to exert analgesic effects in chronic nerve and muscle pain. Cyclobenzaprine may have a similar effect.
Others contend that the structure is more closely related to cyproheptadine, an antagonist at histamine H1 receptors, muscarinic acetylcholine receptors, and 5-HT2A serotonin receptors. Corroborating studies show that cyclobenzaprine causes inhibition of descending serotonergic systems in the spinal cord by blocking 5-HT2A and 5-HT2C receptors. This action is thought to have an inhibitory effect on the alpha motor neurons in the ventral horn of the spinal cord, thereby resulting in decreased firing of alpha-motor neurons and a reduction in spinal mono- and polysynaptic reflexes.
After sustaining an injury, muscle spasms may occur to stabilize the affected body part and prevent further damage. They also generate pain. Cyclobenzaprine is FDA-approved to treat such muscle spasm associated with acute, painful musculoskeletal conditions. It decreases pain in the first two weeks, peaking in the first few days, but has no proven benefit after two weeks. Since no benefit is proven beyond that, therapy should not be continued long-term. It is not useful for spasticity due to neurologic conditions such as cerebral palsy.
Cyclobenzaprine has also shown effectiveness in the treatment of fibromyalgia symptoms. Like other tricyclic antidepressants, it is also prescribed off-label as a sleep-aid.
Cyclobenzaprine is sometimes used for recreational purposes, where it is often referred to as “cyclone” or “mellow yellow.” Users report mild to moderate drowsiness and relaxation as the primary effects.
Formulations and dosages
Cyclobenzaprine hydrochloride is a white, crystalline tricyclic amine salt with the empirical formula C20H21N•HCl and a molecular weight of 311.9. It has a melting point of 217°C, and a pKa of 8.47 at 25°C. It is freely soluble in water and alcohol, sparingly soluble in isopropanol, and insoluble in hydrocarbon solvents. If aqueous solutions are made alkaline, the free base separates. Cyclobenzaprine HCl is designated chemically as 3-(5H-dibenzo[a,d] cyclohepten-5-ylidene)-N, N-dimethyl-1-propanamine hydrochloride, and has the following structural formula:
FLEXERIL 5 mg (Cyclobenzaprine HCl) is supplied as a 5 mg tablet for oral administration. FLEXERIL 10 mg (Cyclobenzaprine HCl) is supplied as a 10 mg tablet for oral administration.
FLEXERIL tablets contain the following inactive ingredients: hydroxypropyl cellulose, hydroxypropyl methylcellulose, iron oxide, lactose, magnesium stearate, starch, and titanium dioxide. FLEXERIL 5 mg tablets also contain Yellow D&C #10 Aluminum Lake HT, and Yellow FD&C #6 Aluminum Lake.
Cyclobenzaprine is marketed as Apo-Cyclobenzaprine (10 mg tablets), Flexeril (5 and 10 mg tablets) and Fexmid (7.5 mg tablet). Both Flexeril and Fexmid are available in generic form. A once-a-day extended-release formulation Amrix is available in 15- and 30-mg capsules.
Cyclobenzaprine is regulated in the U.S. for prescription use only. It does not fall within most governmental guidelines as a controlled substance, however possession without a valid or current prescription may be illegal, depending upon various state and local laws.
Flexeril side effects
Get emergency medical help if you have any of these signs of an allergic reaction: hives; difficulty breathing; swelling of your face, lips, tongue, or throat. Stop using Flexeril and call your doctor at once if you have any of these serious side effects:
- fast, pounding, or uneven heartbeats;
- chest pain or heavy feeling, pain spreading to the arm or shoulder, nausea, sweating, general ill feeling;
- sudden numbness or weakness, especially on one side of the body;
- sudden headache, confusion, problems with vision, speech, or balance;
- feeling light-headed, fainting;
- confusion, weakness, lack of coordination;
- nausea, stomach pain, low fever, loss of appetite, dark urine, clay-colored stools, jaundice (yellowing of the skin or eyes);
- seizure (convulsions);
- unusual thoughts or behavior, hallucinations (seeing things); or
- easy bruising or bleeding, unusual weakness.
Less serious side effects may include:
- dry mouth or throat;
- blurred vision;
- drowsiness, dizziness, tired feeling;
- loss of appetite, stomach pain, nausea;
- diarrhea, constipation, gas; or
- muscle weakness.
This is not a complete list of side effects and others may occur. Tell your doctor about any unusual or bothersome side effect. You may report side effects to FDA at 1-800-FDA-1088.
SIDE EFFECTS
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: 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:
| 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: 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.
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.
‡ Note: FLEXERIL 10 mg data are from one clinical trial. FLEXERIL 5 mg and placebo data are from two studies.
DRUG INTERACTIONS
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
What other drugs will affect Flexeril?
Many drugs can interact with Flexeril. Below is just a partial list. Tell your doctor if you are using:
- atropine (Donnatal, and others), benztropine (Cogentin), dimenhydrinate (Dramamine), methscopolamine (Pamine), or scopolamine (Transderm-Scop);
- a bronchodilator such as ipratroprium (Atrovent) or tiotropium (Spiriva);
- glycopyrrolate (Robinul);
- guanethidine (Ismelin);
- mepenzolate (Cantil);
- tramadol (Ultram);
- bladder or urinary medications such as darifenacin (Enablex), flavoxate (Urispas), oxybutynin (Ditropan, Oxytrol), tolterodine (Detrol), or solifenacin (Vesicare); or
- irritable bowel medications such as dicyclomine (Bentyl), hyoscyamine (Anaspaz, Cystospaz, Levsin, and others), or propantheline (Pro-Banthine).
This list is not complete and there may be other drugs that can interact with Flexeril. Tell your doctor about all your prescription and over-the-counter medications, vitamins, minerals, herbal products, and drugs prescribed by other doctors. Do not start a new medication without telling your doctor.
PRECAUTIONS
General
Because of its atropine-like action, FLEXERIL should be used with caution in patients with a history of urinary retention, angle-closure glaucoma, increased intraocular pressure, and in patients taking anticholinergic medication.
Impaired Hepatic Function
The plasma concentration of cyclobenzaprine is increased in patients with hepatic impairment.
These patients are generally more susceptible to drugs with potentially sedating effects, including cyclobenzaprine. FLEXERIL should be used with caution in subjects with mild hepatic impairment starting with a 5 mg dose and titrating slowly upward. Due to the lack of data in subjects with more severe hepatic insufficiency, the use of FLEXERIL in subjects with moderate to severe impairment is not recommended.
Carcinogenesis, Mutagenesis, Impairment of Fertility
In rats treated with FLEXERIL for up to 67 weeks at doses of approximately 5 to 40 times the maximum recommended human dose, pale, sometimes enlarged, livers were noted and there was a dose-related hepatocyte vacuolation with lipidosis. In the higher dose groups this microscopic change was seen after 26 weeks and even earlier in rats which died prior to 26 weeks; at lower doses, the change was not seen until after 26 weeks.
Cyclobenzaprine did not affect the onset, incidence or distribution of neoplasia in an 81-week study in the mouse or in a 105-week study in the rat.
At oral doses of up to 10 times the human dose, cyclobenzaprine did not adversely affect the reproductive performance or fertility of male or female rats. Cyclobenzaprine did not demonstrate mutagenic activity in the male mouse at dose levels of up to 20 times the human dose.
Pregnancy
Pregnancy Category B: Reproduction studies have been performed in rats, mice and rabbits at doses up to 20 times the human dose, and have revealed no evidence of impaired fertility or harm to the fetus due to FLEXERIL. There are, however, no adequate and well-controlled studies in pregnant women. Because animal reproduction studies are not always predictive of human response, this drug should be used during pregnancy only if clearly needed.
Nursing Mothers
It is not known whether this drug is excreted in human milk. Because cyclobenzaprine is closely related to the tricyclic antidepressants, some of which are known to be excreted in human milk, caution should be exercised when FLEXERIL is administered to a nursing woman.
Pediatric Use
Safety and effectiveness of FLEXERIL in pediatric patients below 15 years of age have not been established.
Use in the Elderly
The plasma concentration of cyclobenzaprine is increased in the elderly. The elderly may also be more at risk for CNS adverse events such as hallucinations and confusion, cardiac events resulting in falls or other sequelae, drug-drug and drug-disease interactions. For these reasons, in the elderly, cyclobenzaprine should be used only if clearly needed. In such patients FLEXERIL should be initiated with a 5 mg dose and titrated slowly upward.
