What Is Cyclobenzaprine (Flexeril)? Uses, Side Effects & Warnings

Cyclobenzaprine is a centrally-acting skeletal muscle relaxant available by prescription. Best known by its original brand name Flexeril, it is also sold as Amrix (extended-release capsules). Immediate-release tablets come in 5 mg and 10 mg strengths; extended-release capsules come in 15 mg and 30 mg. It was first approved by the FDA in 1977 and remains one of the most commonly prescribed muscle relaxants in the United States, typically used alongside rest and physical therapy for acute, painful musculoskeletal conditions.

What Cyclobenzaprine Is Used For

Cyclobenzaprine is FDA-approved as an adjunct to rest and physical therapy for the relief of muscle spasm associated with acute, painful musculoskeletal conditions in adults. The operative word here is "acute" — the drug is designed for short-term use only, typically no longer than two to three weeks. It is effective for relieving the pain, tenderness, and limitation of motion caused by muscle injuries such as strains, sprains, and muscle spasms from back injuries.

Cyclobenzaprine has no FDA-approved indication for spasticity (the chronic increase in muscle tone seen in neurological conditions like multiple sclerosis, stroke, or cerebral palsy) — a distinction that is clinically important. Drugs like baclofen and tizanidine are preferred for that type of muscle problem.

How Cyclobenzaprine Works

Despite being classified as a muscle relaxant, cyclobenzaprine does not act directly on skeletal muscle or at the neuromuscular junction. Instead, it works primarily within the central nervous system — specifically in the brainstem — where it is thought to reduce tonic somatic motor activity by influencing descending pathways from the brain to the spinal cord.

Structurally, cyclobenzaprine is closely related to the tricyclic antidepressants (TCAs), particularly amitriptyline. It differs from amitriptyline by only one double bond in its molecular structure. This structural similarity means cyclobenzaprine shares many of the same pharmacological properties as TCAs — including antihistamine activity (causing sedation), anticholinergic activity (causing dry mouth, urinary retention, constipation, and blurred vision), and alpha-adrenergic blockade (causing blood pressure drops). These TCA-like properties explain much of the drug's side effect profile and its contraindications.

Common Side Effects

• Drowsiness and sedation (very common — affects the majority of patients)
• Dry mouth
• Dizziness
• Fatigue
• Headache
• Constipation
• Blurred vision
• Nausea
• Difficulty urinating (urinary retention) — especially in older men with enlarged prostates
• Confusion or impaired thinking, particularly in older adults

Important Warnings

Cyclobenzaprine should not be used in patients with hyperthyroidism — it can trigger arrhythmias in this context. It should also be used with caution in patients with angle-closure glaucoma or urinary retention, due to its anticholinergic properties.

Key Drug Interactions

The most critical interaction is with MAO inhibitors (see warning above). Beyond that, cyclobenzaprine's CNS depressant effects compound with alcohol, benzodiazepines, opioids, and other sedating medications — increasing the risk of over-sedation, respiratory depression, and accidents. The combination with tramadol carries a risk of seizures. Because of its serotonergic properties, combining cyclobenzaprine with SSRIs, SNRIs, or other serotonergic drugs may increase the risk of serotonin syndrome. Anticholinergic drugs (including some antihistamines, bladder medications, and older antidepressants) can have additive anticholinergic effects when combined with cyclobenzaprine.

Frequently Asked Questions

Is cyclobenzaprine a controlled substance?

Cyclobenzaprine is not currently classified as a federally controlled substance in the United States. However, it does have abuse potential — its sedating effects make it a drug of misuse for some individuals, often in combination with other CNS depressants. Some states have placed additional scheduling restrictions on it. It should only be used as prescribed and should be stored securely.

Can cyclobenzaprine be used for fibromyalgia?

Low-dose cyclobenzaprine has been studied for fibromyalgia — a chronic pain and fatigue condition — and some trials have shown modest improvements in sleep quality and pain. However, it is not FDA-approved for this use. The dose used in fibromyalgia research is typically lower than what is used for acute muscle spasms, and it is given at bedtime to take advantage of the sedating effect.

Why can't cyclobenzaprine be used long-term?

Clinical evidence for efficacy beyond 2–3 weeks is lacking, and the drug's side effect profile — particularly sedation and anticholinergic effects — becomes problematic with sustained use. Tolerance to the sedating effects may develop, and long-term anticholinergic exposure is associated with cognitive decline and worsening urinary and GI function. For chronic muscle problems, other approaches (physical therapy, targeted exercises, other medications) are more appropriate.

Will cyclobenzaprine show up on a drug test?

Standard urine drug screens do not test specifically for cyclobenzaprine. However, it can occasionally produce false positives for tricyclic antidepressants on some immunoassay tests, due to its structural similarity to TCAs. Confirmatory testing (GC-MS) can distinguish between the two.