What is Flexeril? Understanding Uses, Dosage, and Side Effects

Cyclobenzaprine, commonly known by the brand name Flexeril, is a medication primarily used as an adjunct to rest and physical therapy for the short-term relief of muscle spasms associated with acute, painful musculoskeletal conditions. It belongs to a class of drugs called cyclical antidepressants, although its primary use is as a muscle relaxant. Understanding what Flexeril is, how it works, its potential side effects, and contraindications is crucial for both healthcare professionals and patients.

Flexeril Uses: What Conditions Does It Treat?

Flexeril is FDA-approved for the treatment of muscle spasms, but it also has some off-label uses.

• FDA-Approved Indication: Short-term relief of muscle spasms associated with acute musculoskeletal conditions.

• Off-label Clinical Uses: Although not officially approved, Flexeril has been used for conditions such as fibromyalgia, temporomandibular joint (TMJ) disorders, and, in some studies, for military-related PTSD.

How Does Flexeril Work? Understanding the Mechanism of Action

Alt text: Diagram illustrating the mechanism of action of Cyclobenzaprine within the central nervous system.

Flexeril works as a centrally acting skeletal muscle relaxant. It reduces muscle hyperactivity primarily within the central nervous system, specifically in the brain stem. It doesn’t directly act on skeletal muscles or the neuromuscular junction, but rather decreases tonic somatic motor activity, affecting both gamma (γ) and alpha (α) motor systems. Recent research suggests that Flexeril acts as a (5-HT2) receptor antagonist, contributing to its antispasmodic effect.

Pharmacokinetics:

• Absorption: Peak plasma concentration is reached in about 4 hours for the immediate-release formulation and 7 to 8 hours for the extended-release version. Food can increase the absorption rate.
• Distribution: Flexeril is highly bound to plasma proteins (93%), primarily to alpha-1 glycoprotein.
• Metabolism: It is metabolized by cytochrome P450 enzymes (CYP3A4, CYP1A2, and CYP2D6).
• Excretion: The mean elimination half-life is approximately 18 hours for the immediate-release formulation and 32 hours for the extended-release. It is primarily excreted as glucuronides via the kidney.

Flexeril Dosage and Administration

Flexeril is administered orally and is available in immediate-release tablets (5 mg, 7.5 mg, and 10 mg) and extended-release capsules (15 mg and 30 mg).

• Immediate-release tablets are typically taken three times daily, with a maximum recommended dose of 30 mg per day.
• Extended-release capsules should be taken at the same time each day. The capsule can be swallowed whole or the contents sprinkled onto a tablespoon of applesauce for immediate consumption.

Use in Specific Patient Populations:

• Hepatic Impairment: Use with caution in patients with mild hepatic impairment. A starting dose of 5 mg is recommended, with titration upwards if needed. It is not recommended in moderate to severe hepatic impairment.
• Renal Impairment: Avoid use in chronic kidney disease, especially in elderly patients, due to potential anticholinergic effects, sedation, and increased fracture risk.
• Pregnancy: Use during pregnancy only if clearly needed, as there is limited clinical data on its safety.
• Breastfeeding: Breastfeeding can be continued with caution if Flexeril is indicated for the mother, but the infant should be monitored for sedation and developmental milestones.

Flexeril Side Effects: What Are the Potential Adverse Reactions?

The most common adverse effects of Flexeril include:

• Somnolence (drowsiness)
• Dry mucous membranes
• Dizziness
• Confusion

As a cyclical antidepressant, Flexeril can also antagonize muscarinic receptors, leading to side effects such as xerostomia, ileus, tachycardia, mydriasis, urinary retention, and hallucinations. It may also cause vasodilation and reflex tachycardia due to alpha1 adrenergic receptor antagonism. Chronic use may cause minor ALT elevation, but severe hepatotoxicity is rare.

Flexeril Contraindications: When Should You Avoid This Medication?

Flexeril is contraindicated in patients with:

• Prior hypersensitivity reactions
• Hyperthyroidism
• Acute recovery phase of myocardial infarction
• Arrhythmias
• Heart failure
• Heart block or conduction disturbances
• Concurrent use or within 14 days of taking a monoamine oxidase inhibitor (MAOI)

Monitoring for Serotonin Syndrome and Other Complications

Clinicians should monitor patients for signs and symptoms of serotonin syndrome, especially when taking other serotonergic drugs. Flexeril can also cause reflex tachycardia, so vital signs should be monitored. Pain levels should be assessed using tools like the Numerical Rating Scale (NRS), Verbal Rating Scale (VRS), Visual Analogue Scale (VAS), or Faces Pain Scale-Revised (FPS-R). For TMJ disorders, a graded chronic pain scale (GCPS) can be used. Regular evaluation of the necessity of Flexeril use is important to avoid overprescription.

Flexeril Overdose and Toxicity

Alt text: EKG image illustrating the QRS widening commonly observed in Cyclobenzaprine toxicity cases.

Flexeril is structurally and pharmacologically related to tricyclic antidepressants, and overdoses can affect cardiac sodium channels, leading to QRS widening on electrocardiograms. It may also decrease the seizure threshold. Common effects of overdose include drowsiness and tachycardia. Severe manifestations can include cardiac arrest, cardiac dysrhythmias, severe hypotension, seizures, and neuroleptic malignant syndrome.

Management of Toxicity:

• Ensure airway, breathing, and circulation.
• Obtain an EKG and initiate cardiac monitoring.
• Protect the patient’s airway and establish an intravenous line.
• Perform gastrointestinal decontamination with gastric lavage followed by activated charcoal.
• Administer serum alkalinization using sodium bicarbonate for QRS prolongation.
• Treat dysrhythmias unresponsive to sodium bicarbonate with phenytoin, lidocaine, or bretylium.
• Control seizures with benzodiazepines or other anticonvulsants.
• Consider physostigmine administration in consultation with a poison control center.

The Importance of an Interprofessional Approach

Managing patients on Flexeril requires collaboration among healthcare providers, including physicians, dentists, pharmacists, nurses, and physical therapists. Effective communication using tools like SBAR (situation, background, assessment, and recommendation) can improve patient safety and outcomes. An interprofessional team approach can lead to improved efficacy, minimized adverse drug reactions, and increased patient satisfaction.

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