Cyclobenzaprine
Flexeril (IR), Amrix (ER), Tonmya (sublingual)
Cyclobenzaprine Indications
| Indication | Approved Population | Therapy Type | Status |
|---|---|---|---|
| Muscle spasm associated with acute, painful musculoskeletal conditions | Adults and adolescents ≥15 years | Adjunct to rest and physical therapy | FDA Approved |
| Fibromyalgia (sublingual formulation only — Tonmya) | Adults ≥18 years | Monotherapy (bedtime dosing) | FDA Approved (Aug 2025) |
Cyclobenzaprine is the most widely studied centrally acting muscle relaxant. For acute musculoskeletal spasm, it is intended for short-term use (2–3 weeks) only, as clinical trials have not demonstrated benefit beyond this period and because most acute musculoskeletal conditions are self-limited. It is ineffective for spasticity arising from central nervous system disorders such as cerebral palsy or spinal cord injury. In August 2025, the FDA approved Tonmya (cyclobenzaprine HCl sublingual tablets) for fibromyalgia in adults, making it the first new treatment approved for this condition in over 15 years.
Low-dose for fibromyalgia-related sleep disturbance (oral IR) — Low-dose cyclobenzaprine (5–10 mg at bedtime) has been used off-label for fibromyalgia-associated sleep disruption. Several randomized trials demonstrate modest benefit for sleep quality and pain. Evidence quality: Moderate. Note that the approved sublingual formulation (Tonmya) is now the preferred route for this indication.
Temporomandibular joint disorder (TMD) — Some clinicians prescribe cyclobenzaprine for jaw muscle spasm and myofascial pain related to TMD. Evidence is limited to small trials and case series. Evidence quality: Low.
Cyclobenzaprine Dosing
Adult Dosing — By Clinical Scenario
| Clinical Scenario | Starting Dose | Maintenance Dose | Maximum Dose | Notes |
|---|---|---|---|---|
| Acute musculoskeletal spasm — standard therapy (IR) | 5 mg TID | 5–10 mg TID | 30 mg/day | Limit to 2–3 weeks; reassess if no improvement Onset within 1 h; some clinicians dose at bedtime only if sedation is problematic |
| Acute musculoskeletal spasm — once-daily dosing (ER) | 15 mg once daily | 15–30 mg once daily | 30 mg/day | Take at approximately the same time each day Swallow capsule whole; may sprinkle on applesauce if unable to swallow |
| Fibromyalgia — sublingual (Tonmya) | 2.8 mg SL at bedtime | 5.6 mg SL at bedtime | 5.6 mg/day | 2.8 mg for first 14 days, then increase to 5.6 mg (two 2.8 mg tablets) Place under tongue at bedtime; do not chew or swallow tablets |
| Elderly patient — acute spasm (IR only) | 5 mg once daily at bedtime | 5 mg TID | 15 mg/day | Titrate slowly; higher risk of confusion, falls, and anticholinergic effects ER formulation (Amrix) is not recommended in the elderly; Tonmya not studied in ≥65 years |
| Hepatic impairment — mild (IR only) | 5 mg once daily | 5 mg TID | 15 mg/day | AUC approximately doubled in mild hepatic impairment Avoid IR in moderate-to-severe impairment; ER not recommended at any level of hepatic impairment |
| Fibromyalgia — mild hepatic impairment or geriatric (SL) | 2.8 mg SL at bedtime | 2.8 mg SL at bedtime | 2.8 mg/day | Do not escalate to 5.6 mg in these populations (Tonmya PI) |
For acute musculoskeletal spasm, cyclobenzaprine should be limited to 2–3 weeks. Clinical trials have not demonstrated continued benefit beyond this timeframe, and the underlying conditions are typically self-limited. In contrast, the sublingual formulation for fibromyalgia (Tonmya) is intended for chronic use, with clinical trials evaluating safety over 12 months. These are fundamentally different treatment paradigms despite sharing the same active molecule.
Pharmacology
Mechanism of Action
Cyclobenzaprine is structurally related to the tricyclic antidepressants amitriptyline and imipramine, differing from amitriptyline by only a single double bond in the central ring. It acts primarily within the central nervous system at the brainstem level, rather than at the spinal cord or neuromuscular junction, to reduce tonic somatic motor activity. The net pharmacological effect is a reduction in muscle hyperactivity through modulation of both gamma and alpha motor neuron systems. Cyclobenzaprine demonstrates functional antagonism at 5-HT2A serotonergic, alpha1-adrenergic, H1-histaminergic, and M1-muscarinic receptors. Its serotonin receptor antagonism and influence on descending serotonergic pathways are thought to underlie its efficacy in fibromyalgia, where it targets nonrestorative sleep architecture and central pain processing.
ADME Profile
| Parameter | Value | Clinical Implication |
|---|---|---|
| Absorption | Oral bioavailability 33–55%; Tmax ~4 h (IR), 7–8 h (ER); food increases AUC | Variable absorption contributes to interpatient variability in response; ER capsule provides sustained levels over 24 h |
| Distribution | Vd ~146 L; 93% protein-bound (primarily alpha1-acid glycoprotein) | Extensive tissue distribution; binding to alpha1-acid glycoprotein (an acute-phase reactant) means protein binding may increase during inflammation |
| Metabolism | Hepatic via CYP3A4 and CYP1A2 (major), CYP2D6 (minor); N-demethylation to norcyclobenzaprine; glucuronidation via UGT1A4/2B10; enterohepatic circulation | CYP3A4 and CYP1A2 inhibitors may increase exposure; sublingual route bypasses first-pass metabolism, reducing norcyclobenzaprine formation |
| Elimination | t1/2 18 h (IR, range 8–37 h), ~32–35 h (ER); clearance 0.7 L/min; primarily renal as glucuronide conjugates | Long half-life causes accumulation at steady state (~4-fold with TID dosing); substantially prolonged in elderly (AUC ~1.7-fold higher) and hepatic impairment (AUC ~2-fold higher) |
Side Effects
| Adverse Effect | Incidence | Clinical Note |
|---|---|---|
| Drowsiness / somnolence | 29% (IR 5 mg); 38% (IR 10 mg) | Most common reason for discontinuation; dose-dependent; placebo rate ~10%; ER shows lower rate (~2% at 30 mg in controlled trials) (FDA PI) |
| Dry mouth | 21% (IR 5 mg); 32% (IR 10 mg); 14% (ER 30 mg); 6% (ER 15 mg) | Anticholinergic effect; dose-dependent across all formulations; oral hygiene counselling important; risk of dental caries with prolonged use |
| Dizziness | 11% (IR 10 mg clinical studies) | Dose-dependent; substantially lower at IR 5 mg (1–3%) and ER formulations (3–6%); contributes to fall risk in elderly |
| Adverse Effect | Incidence | Clinical Note |
|---|---|---|
| Fatigue | 6% (IR 5 & 10 mg); 3% (ER) | Usually improves within the first week; may be less pronounced with ER formulation |
| Headache | 5% (IR) | Comparable to placebo rate (8%) in controlled trials; not clearly drug-related |
| Constipation | 1–3% (IR); 3% (ER 30 mg) | Anticholinergic mechanism; increase fibre and fluid intake |
| Nausea | 1–3% (IR); 3% (ER) | Take with food if gastrointestinal upset occurs |
| Dyspepsia | 1–3% | Usually mild and self-limited |
| Blurred vision | 1–3% | Anticholinergic effect; warn patients about driving impairment |
| Unpleasant taste | 1–3% | More common with IR formulation; transient |
Tonmya-specific (sublingual): Oral hypoesthesia (23%), abnormal product taste (11%), oral paresthesia (7%), oral discomfort, oral pain, and aphthous ulcer were common local effects in the RELIEF and RESILIENT trials. These were transient and self-limited.
| Adverse Effect | Estimated Frequency | Typical Onset | Required Action |
|---|---|---|---|
| Serotonin syndrome | Rare | Hours to days after initiation or dose increase, especially with concomitant serotonergic drugs | Discontinue cyclobenzaprine and all serotonergic agents immediately; supportive care; cyproheptadine may be considered |
| Cardiac arrhythmias (tachycardia, conduction disturbances, QRS prolongation) | Rare (<1%) | Any time during therapy; higher risk with overdose or in patients with pre-existing cardiac disease | ECG monitoring; discontinue if QRS widening; cardiology consultation; sodium bicarbonate for significant QRS prolongation |
| Anaphylaxis / angioedema | Very rare | Any time, typically early in therapy | Permanent discontinuation; emergency treatment with epinephrine |
| Seizures | Very rare | Primarily in overdose; may occur at therapeutic doses with predisposing factors | Discontinue; benzodiazepines for acute seizure management; neurology referral |
| Hepatitis / cholestasis | Very rare (case reports) | Weeks to months | Discontinue; monitor liver function tests; hepatology referral if LFTs > 3x ULN |
| Neuroleptic malignant syndrome | Very rare (overdose) | Primarily reported in overdose settings | Emergency care; discontinue all causative agents; dantrolene and supportive measures |
| Reason for Discontinuation | Incidence | Context |
|---|---|---|
| Drowsiness / somnolence | Most common | Dose-related; 5 mg dose associated with significantly lower sedation vs 10 mg |
| Dry mouth | Common | More frequent at higher doses; often tolerable with oral care |
| Dizziness | Less common | Usually dose-related; resolves with dose reduction or discontinuation |
Drowsiness affects up to 38% of patients at the 10 mg TID dose but is substantially lower at 5 mg TID (29%) and with the ER formulation. Strategies include starting at the lowest effective dose, administering the largest portion at bedtime, and using the ER formulation for patients who need once-daily convenience with a smoother pharmacokinetic profile. Advise patients that sedation typically improves within the first 3–4 days of treatment. If somnolence remains intolerable, the 5 mg dose three times daily is often sufficient for clinical benefit.
Drug Interactions
Cyclobenzaprine is metabolized primarily by CYP3A4 and CYP1A2, with minor contribution from CYP2D6. Its structural similarity to tricyclic antidepressants means it shares many of the same interaction liabilities, particularly with serotonergic drugs, MAO inhibitors, and CNS depressants. The sublingual formulation bypasses first-pass metabolism but retains systemic interaction potential.
Monitoring
-
Symptom Response
At 1–2 weeks, then at 3 weeks
Routine Assess pain relief, muscle spasm resolution, and functional improvement. If no benefit after 2–3 weeks, discontinue (for acute musculoskeletal indication). For fibromyalgia (Tonmya), assess at weeks 2, 4, and 14 per clinical trial design. -
CNS Effects
Each visit
Routine Monitor for excessive drowsiness, dizziness, confusion, and cognitive impairment, especially in elderly patients. Assess ability to drive and operate machinery safely. -
Hepatic Function
Baseline (if risk factors); as clinically indicated
Trigger-based Obtain baseline liver function tests in patients with known hepatic disease. Repeat if symptoms of hepatotoxicity develop (jaundice, dark urine, unexplained abdominal pain). Drug is not recommended in moderate-to-severe hepatic impairment. -
Cardiac Status
Baseline ECG if cardiac risk factors
Trigger-based Consider baseline ECG in patients with pre-existing cardiac conduction abnormalities or those on other QT-prolonging medications. Monitor for palpitations, tachycardia, or syncope during therapy. -
Anticholinergic Burden
Baseline and periodically
Routine Assess for urinary retention, constipation, dry mouth, blurred vision, and cognitive effects. Particularly important when combined with other anticholinergic medications; use anticholinergic burden scoring tools in elderly. -
Serotonin Syndrome Signs
After initiation or dose change with concurrent serotonergic drugs
Trigger-based Monitor for agitation, hyperthermia, diaphoresis, tachycardia, hyperreflexia, clonus, and altered mental status if cyclobenzaprine is co-prescribed with SSRIs, SNRIs, triptans, or tramadol.
Contraindications & Cautions
Absolute Contraindications
- Hypersensitivity to cyclobenzaprine or any component of the formulation
- Concomitant MAO inhibitor use or use within 14 days of MAOI discontinuation — risk of hyperpyretic crisis, seizures, and death
- Acute recovery phase of myocardial infarction
- Cardiac arrhythmias, heart block, or conduction disturbances
- Congestive heart failure
- Hyperthyroidism
Relative Contraindications (Specialist Input Recommended)
- Moderate-to-severe hepatic impairment — drug is not recommended due to substantially increased plasma concentrations and lack of safety data
- Elderly patients (≥65 years) — Beers Criteria lists cyclobenzaprine as potentially inappropriate; ER formulation is not recommended; increased risk of falls, confusion, delirium, and anticholinergic effects
- Concurrent use of multiple serotonergic agents — risk-benefit assessment required due to serotonin syndrome risk; consider alternative muscle relaxant
Use with Caution
- Mild hepatic impairment — start IR at 5 mg with slow titration; ER not recommended
- History of urinary retention or angle-closure glaucoma — anticholinergic properties may worsen these conditions
- Increased intraocular pressure — monitor if pre-existing ocular hypertension
- Patients taking anticholinergic medications — additive anticholinergic burden
- Patients operating heavy machinery or driving — impaired mental and physical abilities, especially early in therapy
Cyclobenzaprine, due to its structural and pharmacological similarity to tricyclic antidepressants, carries a risk of serotonin syndrome when used with other serotonergic agents. Cases have been reported when combined with SSRIs, SNRIs, triptans, and MAO inhibitors. Clinicians should be vigilant for the triad of altered mental status, autonomic instability, and neuromuscular excitability. The combination with MAO inhibitors is an absolute contraindication due to reports of fatal hyperpyretic crises.
Patient Counselling
Purpose of Therapy
Cyclobenzaprine is a muscle relaxant used alongside rest and physical therapy to relieve muscle spasm and the associated pain, tenderness, and limited movement that accompany acute musculoskeletal injuries. For the sublingual formulation (Tonmya), the medication is used specifically for fibromyalgia, targeting chronic pain and sleep disturbance. It is important to explain that for acute muscle spasm, the medication is intended for short-term use (2–3 weeks) only, whereas the fibromyalgia indication involves ongoing treatment.
How to Take
Immediate-release tablets: Swallow whole with water. Take at evenly spaced intervals throughout the day, or as directed. If drowsiness is significant, the prescriber may adjust timing so the larger portion is taken at bedtime. Extended-release capsules: Swallow whole at approximately the same time each day. If unable to swallow the capsule, it may be opened and the contents sprinkled on a tablespoon of applesauce and swallowed immediately without chewing. Sublingual tablets (Tonmya): Place under the tongue at bedtime and allow to dissolve completely. Do not chew or swallow the tablet. Do not eat or drink for at least 10 minutes after administration.
Sources
- Flexeril (cyclobenzaprine hydrochloride) tablets. Full Prescribing Information. McNeil Consumer & Specialty Pharmaceuticals. FDA Label (NDA 17-821/S-045) Primary source for IR formulation dosing, adverse reactions with incidence rates (5 mg vs 10 mg), contraindications, and pharmacokinetic parameters.
- Amrix (cyclobenzaprine hydrochloride extended-release capsules). Full Prescribing Information. Teva Pharmaceuticals. Revised May 2024. DailyMed Source for ER formulation dosing (15 mg and 30 mg), adverse reaction rates from Phase 3 trials, and hepatic/elderly precautions.
- Tonmya (cyclobenzaprine hydrochloride sublingual tablets). Full Prescribing Information. Tonix Pharmaceuticals. August 2025. Tonix Pharmaceuticals Source for sublingual formulation dosing for fibromyalgia, local adverse effects (oral hypoesthesia, paresthesia), and embryo-fetal toxicity warnings.
- Darwish M, Hellriegel ET. Steady-state pharmacokinetics of once-daily cyclobenzaprine extended release: a randomized, double-blind, 2-period crossover study in healthy volunteers. Clin Ther. 2011;33(6):746-753. doi:10.1016/j.clinthera.2011.05.043 Characterised ER steady-state PK: Tmax 7 h, t1/2 34.8 h; source for ER pharmacokinetic data cited in the ADME table.
- Lederman S, et al. Pain relief by targeting nonrestorative sleep in fibromyalgia: a phase 3 randomized trial of bedtime sublingual cyclobenzaprine (RELIEF trial). Arthritis Care Res. 2023. NCT04172831. Phase 3 RELIEF trial (n=503) demonstrating significant reduction in daily pain intensity scores vs placebo at 14 weeks with Tonmya.
- Lederman S, et al. Efficacy and safety of TNX-102 SL in patients with fibromyalgia (RESILIENT trial). Pain Med. 2025. NCT05273749. Phase 3 RESILIENT trial (n=457) confirming pain reduction with sublingual cyclobenzaprine (mean change -1.8 vs -1.2 placebo, P<.001).
- Chou R, et al. Nonpharmacologic therapies for low back pain: a systematic review for an American College of Physicians clinical practice guideline. Ann Intern Med. 2017;166(7):493-505. doi:10.7326/M16-2459 ACP guideline recommending muscle relaxants including cyclobenzaprine as a second-line option for acute low back pain when NSAIDs are insufficient.
- 2023 American Geriatrics Society Beers Criteria Update Expert Panel. American Geriatrics Society 2023 Updated AGS Beers Criteria. J Am Geriatr Soc. 2023;71(7):2052-2081. doi:10.1111/jgs.18372 Lists cyclobenzaprine as potentially inappropriate in older adults due to anticholinergic effects, sedation, and fall risk.
- Kobayashi H, Hasegawa Y, Ono H. Cyclobenzaprine, a centrally acting muscle relaxant, acts on descending serotonergic systems. Eur J Pharmacol. 1996;311(1):29-35. doi:10.1016/0014-2999(96)00402-5 Demonstrated cyclobenzaprine’s action on descending serotonergic systems, informing the mechanistic rationale for its use in fibromyalgia.
- Moldofsky H, et al. Effects of bedtime very low dose cyclobenzaprine on symptoms and sleep physiology in patients with fibromyalgia syndrome: a randomized, double-blind, placebo-controlled study. J Rheumatol. 2011;38(12):2653-2663. doi:10.3899/jrheum.110194 Early proof-of-concept study for low-dose sublingual cyclobenzaprine in fibromyalgia, demonstrating improved sleep physiology.
- Brioschi MB, et al. Pharmacokinetics and bioequivalence evaluation of cyclobenzaprine tablets. Biomed Res Int. 2013;2013:281392. doi:10.1155/2013/281392 Comprehensive PK study reporting Vd ~146 L, multicompartment elimination, and Tmax ~4.5 h for IR formulation.
- Khan I, Kahwaji CI. Cyclobenzaprine. In: StatPearls. Treasure Island (FL): StatPearls Publishing; Updated August 2023. NCBI Bookshelf Comprehensive clinical review covering pharmacology, PK parameters (protein binding 93%, CYP involvement), dosing, and toxicology management.
- Huang Z, Ung T. Effect of alpha-1-acid glycoprotein binding on pharmacokinetics and pharmacodynamics. Curr Drug Metab. 2013;14(2):226-238. doi:10.2174/1389200211314020011 Explains the clinical significance of cyclobenzaprine’s high binding to alpha1-acid glycoprotein and how inflammation may alter drug availability.