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Cyclobenzaprine (Flexeril)

Muscle Relaxantprescriptionoral · predicted

Centrally-acting skeletal muscle relaxant structurally related to tricyclic antidepressants. Used for acute musculoskeletal spasm (typically 2-3 weeks). Acts at the brainstem to reduce tonic somatic motor activity. Metabolized by CYP3A4, CYP1A2, and CYP2D6. Extended-release formulation (Amrix) has ~32h half-life.

Projected serum levels — 10 mg, once daily

Cyclobenzaprine (Flexeril)
Cyclobenzaprine (Flexeril) modeled serum levels, 10 mg once daily over 15 days Population-based pharmacokinetic estimate. Steady state reached after approximately 1 days. 0 2.5 5 7.5 10 Day 0 Day 4 Day 8 Day 11 Day 15 Time on a regular schedule ≈ steady state · day 1
Cyclobenzaprine (Flexeril) modeled serum levels with a loading dose of 12.4 mg, then 10 mg once daily The first dose is larger so levels approach steady state faster. 0 2.5 5 7.5 10 Day 0 Day 4 Day 8 Day 11 Day 15 Time on a regular schedule

Maintenance schedule: 10 mg once daily (oral).

Loading schedule: 12.4 mg on day 1, then 10 mg once daily — reaching therapeutic levels sooner.

Modeled steady state after ~1 days: peak ≈ 5.6 mg, trough ≈ 1.3 mg body load. Population-based estimate over 15 days for a 10 mg dose at a 70 kg reference body mass — the interactive app scales curves to your doses, timing, and body mass.

Key facts

Category
Muscle Relaxant
Route modeled
Oral
Model confidence
predicted
Half-life
18 h
Common dose
10 mg
Suggested maximum
30 mg/day
Reference dose range
5–30 mg (single dose)
Suggested cadence
once daily
Validated against
10 mg oral — Cmax 0.020 mg/L at 4 h

Documented interactions

  • danger
    1P-LSD (1-Propionyl-LSD) + Cyclobenzaprine (Flexeril) Serotonin risk

    1P-LSD (1-Propionyl-LSD) and Cyclobenzaprine (Flexeril) both have serotonergic activity. Risk of serotonin syndrome (hyperthermia, rigidity, autonomic instability, altered mental status).

  • danger
    25I-NBOMe + Cyclobenzaprine (Flexeril) Serotonin risk

    25I-NBOMe and Cyclobenzaprine (Flexeril) both have serotonergic activity. Risk of serotonin syndrome (hyperthermia, rigidity, autonomic instability, altered mental status).

  • danger
    2C-I (2,5-Dimethoxy-4-iodophenethylamine) + Cyclobenzaprine (Flexeril) Serotonin risk

    2C-I (2,5-Dimethoxy-4-iodophenethylamine) and Cyclobenzaprine (Flexeril) both have serotonergic activity. Risk of serotonin syndrome (hyperthermia, rigidity, autonomic instability, altered mental…

  • danger
    2C-B (4-Bromo-2,5-dimethoxyphenethylamine) + Cyclobenzaprine (Flexeril) Serotonin risk

    2C-B (4-Bromo-2,5-dimethoxyphenethylamine) and Cyclobenzaprine (Flexeril) both have serotonergic activity. Risk of serotonin syndrome (hyperthermia, rigidity, autonomic instability, altered mental…

  • danger
    4-AcO-DMT (Psilacetin) + Cyclobenzaprine (Flexeril) Serotonin risk

    4-AcO-DMT (Psilacetin) and Cyclobenzaprine (Flexeril) both have serotonergic activity. Risk of serotonin syndrome (hyperthermia, rigidity, autonomic instability, altered mental status).

  • danger
    5-HTP (5-Hydroxytryptophan) + Cyclobenzaprine (Flexeril) Serotonin risk

    5-HTP (5-Hydroxytryptophan) and Cyclobenzaprine (Flexeril) both have serotonergic activity. Risk of serotonin syndrome (hyperthermia, rigidity, autonomic instability, altered mental status).

Serum checks 205 modeled interaction pairings for cyclobenzaprine (flexeril) across your whole stack — absorption conflicts, enzyme inhibition, and nutrient depletion included.

Research behind this entry

  1. Efficacy of a low-dose regimen of cyclobenzaprine hydrochloride in acute skeletal muscle spasm: results of two placebo-controlled trials Chou R et al. · Clinical Therapeutics, 2003 DOI

    Two RCTs showing cyclobenzaprine 5 mg TID was as effective as 10 mg TID for acute muscle spasm with less sedation, establishing the lower dose as preferred.

  2. Cyclobenzaprine ER for muscle spasm associated with low back and neck pain: two randomized, double-blind, placebo-controlled studies of identical design Malanga GA et al. · Current Medical Research and Opinion, 2009 DOI

    Two identical phase 3 trials demonstrating once-daily extended-release cyclobenzaprine 15-30 mg significantly improved muscle spasm relief versus placebo for low back and neck pain.

  3. Cyclobenzaprine pharmacokinetics, including the effects of age, gender, and hepatic insufficiency Winchell GA et al. · Journal of Clinical Pharmacology, 2002 DOI

    PK study establishing cyclobenzaprine mean half-life of 18 hours (IR) and 32 hours (ER), with significantly prolonged half-life in elderly (33h) and hepatic insufficiency (46h).

3 published studies referenced in the app, each with a plain-language summary.