Generic Name

Flexeril

Mechanism

Flexeril (*cyclobenzaprine*) acts centrally as a short‑acting muscle relaxant. It inhibits the transmission of nerve impulses in the brainstem and spinal cord, producing a relaxation of reflex muscle tone. This is achieved by blocking excitatory neurotransmitters (especially glutamate) and potentiating inhibitory pathways (GABAergic). The result is a reduction of voluntary and involuntary muscle contraction without direct effects on muscle fibers.

Pharmacokinetics

• Absorption – Rapid oral absorption; peak plasma concentration in 1–2 h.
• Distribution – Widely distributed, high protein binding (~92 %).
• Metabolism – Hepatic metabolism via CYP2D6 to hydroxylated metabolites.
• Elimination – Renally excreted (≈80 % unchanged); half‑life 2.6–4.0 h; prolonged in hepatic impairment.
• Drug–drug interactions – Inhibits or is inhibited by CYP2D6 substrates (e.g., fluoxetine, paroxetine).

Indications

• Acute neuromuscular spasm associated with musculoskeletal disorders
• Post‑operative or pre‑operative muscle spasm relief (≤ 3 weeks use)
• Adjunctive therapy for chronic low‑back pain or cervical spine pain when combined with physical therapy

Contraindications

• Hypersensitivity to cyclobenzaprine or phenylpiperazine moiety
• Severe hepatic or renal impairment (dose adjustment required)
• Pregnancy – Category C; use only if benefits outweigh risks
• Breastfeeding – excreted in milk; consider safe alternatives
• Concomitant MAOIs – risk of serotonin syndrome or hypertensive crisis
• Severe cardiac conduction disease – may prolong QT interval
• Geriatric patients – increased sensitivity to CNS depression; start at lowest dose

Dosing

• Take with food to reduce GI upset.
• Taper rapidly if used > 3 weeks to avoid withdrawal.

Adverse Effects

• Common: drowsiness, dry mouth, dizziness, constipation, blurred vision
• Serious: increased seizure risk (especially in epilepsy), marked hypotension, arrhythmias, serotonin syndrome (with serotonergic agents)

Monitoring

• Blood pressure and heart rate on initiation and after dose adjustments
• Mental status for sedation or agitation
• Liver function tests if long‑term use or known hepatic disease
• Pregnancy test in women of childbearing potential
• Kidney function if renally impaired

Clinical Pearls

• Short‑term use only: *Flexeril* is intended for < 3 weeks; chronic pain should rely on multimodal therapy (NSAIDs, physical therapy, CBT).
• Avoid in substance‑misuse: Its benzodiazepine‑like effects can lead to misuse; screen patients with addiction history.
• Taper, don’t stop: Abrupt discontinuation may provoke rebound spasm or withdrawal; taper 5 mg every 3–5 days.
• CYP2D6 genotyping: Poor metabolizers may experience prolonged drug levels, increasing CNS depression.
• Combining with other CNS depressants (alcohol, benzodiazepines, opioids) → additive sedation; use caution and dose reduction.
• Swelling in limbs: If severe edema develops, consider fluid restriction or diuretic introduction as cyclobenzaprine may worsen fluid retention.

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• *This drug card is intended for educational purposes. Always refer to the latest prescribing information and institutional guidelines before clinical use.*