Cyclobenzaprine (Flexeril): Dosage & Side Effects

cyclobenzaprine, flexeril, flexeril dosage
A 15-mg dose of cyclobenzaprine in capsule form.
Credit: National Institutes of Health.

Cyclobenzaprine hydrochloride (known as Flexeril) is a central nervous system muscle relaxant designed to relieve pain and discomfort related to strains, sprains and other muscle injuries. It is usually used in conjunction with rest, physical therapy and other measures. Cyclobenzaprine is not designed for long-term use, and the National Institutes of Medicine recommends that patients not use this drug for more than three weeks without consulting a physician.

Taking cyclobenzaprine

Cyclobenzaprine can be taken as either a tablet or an extended release capsule. The regular tablet’s usual dosage is two to four times a day, while the extended release capsule is generally taken one to two times a day. The tablets, intended for short-term use, come in 5, 7.5, and 10 mg doses. The usual starting dose, according to the DEA, is 5 mg three times a day. The maximum recommended dose is 10 mg three times a day.

According to the FDA, patients should avoid use of monoamine oxidase (MAO) inhibitors within 14 days of cyclobenzaprine use. These include Marplan, Nardil, Parnate and Matulane. Seizures and even death have occurred in patients who have cyclobenzaprine or other structurally similar tricyclic antidepressants with MAO inhibitor drugs.

The FDA recommends cautious use of cyclobenzaprine in patients with a history of urinary retention and glaucoma/eye problems. There is no evidence to suggest that the drug causes any issues for pregnant mothers (according to studies done on mice, rats and rabbits), but there have yet to be any adequate and well-controlled studies in pregnant women. As a result, the FDA cautions use during pregnancy only if clearly needed. For nursing mothers, it is unknown if cyclobenzaprine is excreted in human milk. However, because it is closely related to tricyclic antidepressants, which are excreted in human milk, the FDA cautions against breastfeeding if using cyclobenzaprine.

cyclobenzaprine, flexeril, flexeril dosage
A 10-mg dose of cyclobenzaprine in tablet form.
Credit: National Institutes of Health.

Cyclobenzaprine has not been tested in pediatric patients younger than 15 years of age. For the elderly, they may be at a higher risk for adverse events, including hallucinations and confusion, cardiac events and drug interactions. According to the FDA, the plasma concentration of cyclobenzaprine is increased in the elderly, so the drug should be prescribed only if clearly needed. Initiation should begin with a 5 mg dose and slowly increased, under the careful eye of a doctor.

Side effects

Some common side effects include drowsiness, dry mouth, dizziness, or upset stomach. If these side effects become severe or cause problems in your everyday life, the NIH recommends contacting your doctor.

If you experience severe skin rash, swelling of the face or tongue, difficulty breathing or swallowing, irregular heart rate, chest pain, fever, or seizures, the NIH recommends contacting a doctor immediately. These side effects, which have been reported in other similar tricyclic antidepressants to the FDA, could be indications of arrhythmias, sinus tachycardia, leading to myocardial infarction and stroke.

The abrupt termination of prolonged use can also cause side effects, including nausea, headache, and malaise. However, it is important to note that these withdrawal effects are not indicative of addiction.

cyclobenzaprine, flexeril, flexeril dosage
A 5-mg dose of cyclobenzaprine in tablet form.
Credit: National Institutes of Health.

Abuse and overdose

Cyclobenzaprine may enhance the effects of other central nervous system depressants, such as alcohol, barbiturates, benzodiazepines and narcotics. According to the DEA, abusers often combine cyclobenzaprine with these depressants to produce or enhance psychoactive effects. Though it is not a controlled substance, the DEA has recorded anecdotal reports of use to induce euphoria and relaxation.

Though rare, deaths can occur from cyclobenzaprine overdose, especially in the case of multiple drug ingestion. The FDA recommends that doctors contact a poison control center for current information on treatment for an overdose, as the management of a case is complex. The most common manifestations associated with overdose are drowsiness and tachycardia. Less common effects include tremor, coma, ataxia, hypertension, agitation, slurred speech, nausea, confusion, dizziness, hallucination, and vomiting. Rare but critical effects include cardiac arrest, chest pain and seizures.

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