Cyclobenzaprine
Flexeril
Cyclobenzaprine (Flexeril) is a healing & recovery research compound. Structurally a tricyclic; acts centrally at the brainstem rather than on muscle itself. It doesn't relax the muscle directly — it reduces the tonic descending drive that keeps it spasming.
Cyclobenzaprine quick facts
| Reported research dose | 5–10mg at bedtime |
| Route | Oral |
| Frequency | 1x · As needed |
| Half-life | ~18 hours |
| Forms | Oral |
| Evidence level | Trial-backed for acute muscle spasm; the effect fades after the first week or two |
Useful for acute back or neck spasm, not for chronic use — the benefit is largely front-loaded. Strongly sedating and anticholinergic (dry mouth, grogginess). ⚠️ Not with MAOIs, and additive with alcohol.
How Cyclobenzaprine works
Structurally a tricyclic; acts centrally at the brainstem rather than on muscle itself. It doesn't relax the muscle directly — it reduces the tonic descending drive that keeps it spasming.
Proposed benefits
Researched for soft-tissue and gut repair, reduced inflammation, angiogenesis and faster recovery from injury.
Where to get Cyclobenzaprine
Buy Cyclobenzaprine at AlgoRx →The evidence for Cyclobenzaprine
Graded by what exists behind each claim.
✅ Clinically validated
- Approved with randomized data for acute muscle spasm. Low-dose bedtime formulations have been trialed for fibromyalgia and sleep quality, with modest positive results.
📊 Correlative data
- Wide clinical use. The consistently reported limitation is anticholinergic load — dry mouth, next-day grogginess — which is why low doses at night are the practical pattern.
🧪 Theoretical / extrapolated
- Structurally a tricyclic antidepressant, closely related to amitriptyline, acting centrally rather than on muscle directly. It antagonizes 5-HT2 receptors in the brainstem, reducing descending motor drive.
- Being a TCA predicts the whole side-effect profile — antihistaminergic sedation, anticholinergic dryness — and the serotonergic interaction risk that its 'muscle relaxant' label rather hides.
How to read these tiers: they say how much human evidence exists, not how well something works — and ✗ flags harm, never a disappointing trial. How the evidence tiers work →
What Cyclobenzaprine actually does
Cyclobenzaprine is a tricyclic. Not tricyclic-like — a tricyclic, one double bond in the central seven-membered ring away from amitriptyline. It was developed in the same chemical program and ended up licensed for muscle spasm instead of depression, and the phrase “skeletal muscle relaxant” on the bottle describes the indication, not the pharmacology. Nothing it does happens at the muscle.
The alpha-2 story in most references is wrong, and there is a clean experiment showing it. Kobayashi 1996 measured the monosynaptic reflex in rats. Cyclobenzaprine depressed it dose dependently. The alpha-2 antagonists idazoxan and yohimbine did not block that depression, and destroying noradrenergic neurons with 6-hydroxydopamine did not attenuate it either. What did abolish it was depleting serotonin with DL-p-chlorophenylalanine — and cyclobenzaprine sharply inhibited the reflex facilitation produced by DOI, a 5-HT2 agonist, in spinalized rats. So the target is the 5-HT2 receptor, and the site is the descending serotonergic drive from the brainstem onto the spinal cord. It turns down a facilitatory signal that reaches the motor neuron; it does not touch the contractile apparatus.
And that mechanism is not specific to this molecule. Honda 2003 put cyclobenzaprine, amitriptyline, cyproheptadine and ketanserin through the same preparation and all four inhibited mono- and polysynaptic reflex potentials, all four blocked DOI facilitation, and in all four cases serotonin depletion prevented the effect. Whatever cyclobenzaprine is doing to spasm, three other 5-HT2 antagonists do as well.
The sedation is a separate receptor and it has a number. Singh 2022 cloned the human H1 histamine receptor and found cyclobenzaprine binds it with low nanomolar affinity — and, unusually, non-competitively. Diphenhydramine blocks H1 competitively, so more histamine displaces it. Cyclobenzaprine does not work that way: raising histamine does not out-compete it. That is the pharmacological shape of a sedation you cannot push through, and the label's own numbers say more than a third of people get it — drowsiness 29% at 5 mg and 38% at 10 mg against 10% on placebo US Food and Drug Administration 2013.
Add the muscarinic antagonism the tricyclic scaffold carries (dry mouth 21% at 5 mg, 32% at 10 mg, versus 7% on placebo) and you have the whole drug: a 5-HT2 antagonist for the spasm, an H1 antagonist for the sedation, an anticholinergic for the side effects. One more piece matters for what follows — hepatic N-demethylation produces norcyclobenzaprine, an active metabolite that outlives the parent, and an entire second product exists to avoid it.
Cell, rodent, human — and where it stops
In tissue. Rat spinal cord, drug given systemically, readout the amplitude of the monosynaptic reflex Kobayashi 1996; then the cloned human H1 receptor in a cell line, readout radioligand binding and histamine-driven functional response Singh 2022. Two clean preparations, two clean answers, neither of them a person in pain.
In humans, by mouth, for acute back pain. Browning 2001 pooled the randomized trials. People on cyclobenzaprine were 4.7 times as likely (95% CI 2.7–8.1) to report symptom improvement by day 14 as people on placebo. Read the rest of that sentence, because it is the part that gets dropped: the effect sizes were 0.38 to 0.58 across all five outcomes — modest — the benefit was largest early and declined after the first week, and adverse effects, principally drowsiness, were more common. The review's own conclusion was that shorter courses may be preferable.
In humans, under the tongue, for fibromyalgia — and this is a different drug. The RESILIENT phase 3 randomized 457 adults at 33 US sites to bedtime sublingual cyclobenzaprine or placebo: 2.8 mg for two weeks, then 5.6 mg for twelve, fourteen weeks in total Lederman 2025. The primary endpoint was the change in the daily pain numeric rating scale, and it met it — a least-squares mean reduction of 1.8 points versus 1.2 on placebo — along with all six prespecified key secondary endpoints, including a global-impression responder analysis and the PROMIS sleep-disturbance and fatigue instruments. FDA approved it as Tonmya on 15 August 2025 (company announcement). That is a real approval on a real endpoint, and it is the single most under-reported fact about this molecule.
The obstacle, and it is specific. The oral literature and the sublingual literature are not the same exposure. A 5–10 mg tablet swallowed at bedtime delivers 33–55% of the dose past the liver, carries an effective half-life of 18 hours and accumulates about fourfold on repeat dosing US Food and Drug Administration 2013 Winchell 2002. The sublingual tablet exists to bypass that first pass and to make less norcyclobenzaprine. Nobody has run the two head to head. And a second obstacle sits underneath both: in neither literature was the muscle measured. No electromyographic spasm endpoint, no spasticity scale — every endpoint in every trial is a symptom score a patient wrote down.
Cyclobenzaprine pharmacokinetics — how much of it actually gets in
Route: oral, and slow. Mean oral bioavailability is 33% to 55%, so roughly half of a swallowed dose is removed before it reaches the circulation. Plasma clearance is 0.7 L/min and the effective half-life is 18 hours, with a published range of 8 to 37 hours in eighteen subjects US Food and Drug Administration 2013. Winchell 2002 measured a clearance of 689 mL/min, an absolute bioavailability of 0.55 for a 5 mg dose, linear kinetics from 2.5 to 10 mg three times daily, and about a fourfold accumulation in plasma on multiple dosing.
Do the arithmetic on the 18 hours, because it is the whole next-day problem. Sixteen hours after a bedtime dose — that is 2 p.m. the following afternoon — about 54% of the peak is still there. Take it three nights running and you are dosing on top of roughly four doses' worth of accumulated drug. The prescribing information's two-to-three-week limit is not a caution about dependence; it is a statement that nobody showed benefit past that, while the exposure keeps climbing.
What degrades it. Hepatic oxidation. Cytochromes P450 3A4 and 1A2, and to a lesser extent 2D6, mediate N-demethylation to norcyclobenzaprine; the drug is then excreted primarily as glucuronides via the kidney US Food and Drug Administration 2013. Two consequences follow directly. First, anything that inhibits 3A4 raises exposure of a drug that already accumulates fourfold. Second, the population differences are large and predictable: elderly subjects sit at twice the steady-state plasma concentration of young ones, and mild hepatic insufficiency raises concentrations up to twofold as well Winchell 2002.
The extended-release capsule is the same molecule with the peak flattened rather than the exposure reduced Darwish 2010. The sublingual tablet is the only formulation that changes the metabolite picture, by not sending the dose through the liver first (company announcement).
What would have to be true, and how you would know it was not
Three predictions, and the second one is against the drug.
1. Next-day impairment should track the half-life, not tolerance. If the sedation is non-competitive H1 blockade Singh 2022, it should not fade the way antihistamine grogginess usually does. Test it: 10 mg at 22:00, and a Trail Making B and a week of actigraphy at baseline, day 3 and day 14. Prediction — sleep efficiency rises, and Trail Making B time is still worse than baseline at day 14, most clearly in people over 65, who carry twice the steady-state concentration Winchell 2002. If day-14 psychomotor scores are back to baseline, the non-competitive story is not doing the work it looks like it is doing.
2. The prediction that cuts against it: the pain benefit should stop growing after week one. That is not a hedge, it is Browning 2001's own finding — efficacy greatest early, declining thereafter. So on a 4-week course of 10 mg at bedtime, the week-2 to week-4 change in a daily pain score should be flat while anticholinergic load keeps accruing. Anyone whose pain is still improving in week four is either responding to something else or showing that the 5-HT2 account is incomplete. This is the cheapest measurement on the page and almost nobody makes it.
3. It should tie with amitriptyline, not beat it. Honda 2003 showed cyclobenzaprine, amitriptyline and cyproheptadine are interchangeable on the spinal reflex. A head-to-head against low-dose amitriptyline on the same fibromyalgia pain endpoint should therefore be a draw. If cyclobenzaprine wins, the 5-HT2 mechanism is not the whole mechanism — and the H1 half becomes the candidate explanation.
The safety read-out that belongs in the same window. If you are on an SSRI or SNRI, the thing to watch is not drowsiness but clonus and hyperreflexia, which is what the label's serotonin syndrome warning actually describes US Food and Drug Administration 2013. That is a bedside sign, checkable in thirty seconds, and it is the one that means stop.
What nobody has tested yet
Nobody has run polysomnography inside the trial whose title claims a sleep mechanism. The phase 3 is called “pain relief by targeting nonrestorative sleep” Lederman 2025 and its sleep evidence is a questionnaire. A 30-person crossover with home polysomnography and the same daily pain scale — measuring slow wave sleep and arousal index against next-day pain — would show whether the analgesia is downstream of sleep architecture or simply parallel to it. This is a small, cheap study that would settle the central mechanistic claim of an approved drug.
Nobody has compared 5 mg of the generic tablet with 5.6 mg sublingual. The entire rationale for the branded product is that bypassing the first pass makes less norcyclobenzaprine (company announcement). That is a plausible pharmacokinetic argument and it has never been tested against the cheap tablet on a clinical endpoint. A three-arm trial — sublingual, oral, placebo — with plasma parent and metabolite measured alongside the pain diary would answer it in one study.
Nobody has tested whether non-competitive H1 blockade explains the lack of tolerance to the sedation. Singh 2022 is a receptor-pharmacology paper; the clinical prediction it implies has not been run. Twenty people, a psychomotor battery, single dose versus day 14, cyclobenzaprine against diphenhydramine at equi-sedative doses. If the diphenhydramine arm develops tolerance and the cyclobenzaprine arm does not, the mechanism transfers.
Cyclobenzaprine — its own safety story, not its class's
Read this as a tricyclic, because that is what your body does. The class safety language for a recovery compound does not apply here; the label's own contraindication list does. Cyclobenzaprine is contraindicated with monoamine oxidase inhibitors or within 14 days of stopping one, in the acute recovery phase of myocardial infarction, and in arrhythmias, heart block or conduction disturbances, congestive heart failure and hyperthyroidism US Food and Drug Administration 2013. Those are cardiac and thyroid contraindications on a drug people take for a stiff neck.
Serotonin syndrome is the named, labeled risk — and the people most likely to be co-prescribed this are exactly the people on SSRIs. The label describes mental status change, autonomic instability, and neuromuscular findings including tremor, ataxia, hyperreflexia, clonus and rigidity US Food and Drug Administration 2013. Combining it with an SSRI, an SNRI, tramadol or triptans is the common real-world route into it, and the early sign is neuromuscular, not sedative.
The anticholinergic bill, with numbers. At 10 mg the label reports drowsiness in 38% and dry mouth in 32%, against 10% and 7% on placebo US Food and Drug Administration 2013. Dry mouth is trivial; the same receptor blockade is behind urinary retention, blurred vision, constipation and cognitive fogging, and it is the reason cyclobenzaprine appears on geriatric avoid-lists. The pharmacokinetics make that worse rather than better with age: elderly subjects run at twice the steady-state concentration Winchell 2002, which is why the label says to start the elderly at 5 mg and titrate slowly.
The honest summary of the risk-benefit. The benefit is front-loaded and modest — effect sizes 0.38 to 0.58, fading after week one Browning 2001. The exposure is not front-loaded: it accumulates fourfold and clears with an 18-hour half-life. So every week you continue past the label's two-to-three weeks buys anticholinergic and sedative load with no added analgesia, and that — not addiction, which is not the issue with this drug — is the reason for the short course.
Sources read for this page
- Kobayashi H, Hasegawa Y, Ono H. Cyclobenzaprine, a centrally acting muscle relaxant, acts on descending serotonergic systems.. Eur J Pharmacol 1996 · PMID 8884233
- Honda M, Nishida T, Ono H. Tricyclic analogs cyclobenzaprine, amitriptyline and cyproheptadine inhibit the spinal reflex transmission through 5-HT(2) receptors.. Eur J Pharmacol 2003 · PMID 12498911
- Singh K, Senatorov IS, Cheshmehkani A, Karmokar PF, Moniri NH. The Skeletal Muscle Relaxer Cyclobenzaprine Is a Potent Non-Competitive Antagonist of Histamine H1 Receptors.. J Pharmacol Exp Ther 2022 · PMID 34992159
- Winchell GA. Cyclobenzaprine pharmacokinetics, including the effects of age, gender, and hepatic insufficiency.. J Clin Pharmacol 2002 · PMID 11808825
- Browning R. Cyclobenzaprine and back pain: a meta-analysis.. Arch Intern Med 2001 · PMID 11434793
- Lederman S, Arnold LM, Vaughn B, Engels JM, Kelley M, Sullivan GM. Pain relief by targeting nonrestorative sleep in fibromyalgia: a phase 3 randomized trial of bedtime sublingual cyclobenzaprine.. Pain Med 2025 · PMID 40627411
- Darwish M. Pharmacokinetic profile of once-daily cyclobenzaprine extended-release.. Expert Opin Drug Metab Toxicol 2010 · PMID 20883117
- US Food and Drug Administration. FLEXERIL (cyclobenzaprine HCl) tablets - full prescribing information.. FDA approved labeling, revision 2013
- Tonix Pharmaceuticals Holding Corp. Tonix Pharmaceuticals Announces FDA Approval of Tonmya (cyclobenzaprine HCl sublingual tablets) for the Treatment of Fibromyalgia.. Company announcement, 15 August 2025
Cyclobenzaprine — safety, predicted from mechanism
Predicted from mechanism, not from a human safety trial. How that reasoning works →
What the mechanism predicts
Derived from the molecule, not a trial.
- Repair peptides work by promoting angiogenesis — new blood vessel growth — plus fibroblast migration and growth-factor signaling. That is what makes them useful, and it is the entire basis of the one theoretical concern worth naming: angiogenesis is also what a tumor needs to grow beyond a few millimetres.
- There is no evidence these compounds cause or accelerate cancer. There is a mechanistic reason not to run a pro-angiogenic agent systemically with an active or recently treated malignancy, and that reasoning stands without a trial.
- The second predicted issue is more mundane and more likely: they can mask a signal. Something that reduces pain and inflammation around an injury lets you load a tissue that has not finished healing.
What has actually been reported
- Very well tolerated in reported use. Injection-site reactions and transient light-headedness are the common complaints.
- Human data is thin — most of the literature is rodent — so 'well tolerated' here means 'no signal has emerged from a lot of informal use', which is weaker than a clean trial and stronger than nothing.
How to reduce the risk
Same mechanism as the prediction.
- Stop when the thing you were treating has resolved. There is no mechanism here that demands a clock, and equally no reason to keep running a pro-angiogenic signal once the job is done.
- Do not let reduced pain set your training load. The tissue heals on its own timeline whether or not you can feel it. Reloading early on the strength of feeling better is the most common way people turn a good result into a re-injury.
- Get the diagnosis before the peptide. These accelerate healing of things that heal. A tear that needs surgical repair does not become a tear that does not, and the delay costs you.
- One injury, one compound, long enough to judge it. Otherwise you learn nothing transferable for next time.
What it does to your bloodwork
A fact about the assay.
- Nothing routine tracks these directly. The endpoint is the injury, which means your own honest assessment of function is the measurement.
Don't run this if
- Active or recently treated malignancy — the angiogenesis reasoning.
- Any undiagnosed lump or lesion. Find out what it is first.
The honest unknown
- Long-term systemic exposure in humans has never been characterized. The use case is naturally self-limiting — you stop when the injury resolves — which is why this matters less here than it would elsewhere.
Not medical advice. If you take prescription medication or have a diagnosed condition, check this with a pharmacist or doctor.
Cyclobenzaprine — interference & stacking
Predicted from mechanism, not from an interaction study. How mechanism-predicted claims are made →
What Cyclobenzaprine moves on your bloodwork
Expected direction, not a measured one.
- hs-CRP (High-Sensitivity C-Reactive Protein) — ↓ expected to fall
If a repair peptide is doing anything systemic, inflammation is where it would plausibly show.
What to do: Worth a baseline if you are running one for a chronic issue rather than an acute injury. - Comprehensive Metabolic Panel (CMP) — ◆ worth watching
Standard baseline. Nothing in this class predicts a specific abnormality — which is itself worth saying rather than inventing one.
What to do: Annual is fine unless something changes.
The honest position on this class is that the proliferation question is unresolved — anything that accelerates tissue repair is acting on pathways cancer also uses. Nobody has studied it properly either way.
- How to work up to it, and when not to
- When to take it, and why that window
- Cycle length
- Time off between cycles
- Fasted or fed, and when in the day
- Coach Cam's personal notes
- Which compounds push the same lever, and why the dose adds up faster than people count
- What blunts it — the stacks that waste your money
- What compounds the risk, so a side effect arrives sooner than any one of them suggests
- Coach Cam's read on running it alongside the rest of your protocol
Everything above is free and stays free. Skool is where it becomes a plan — Cyclobenzaprine in an order, with the rest of what you're running.
Unlock in Skool — $10/mo →Bloodwork to run alongside Cyclobenzaprine
Baseline first, then again at 8–12 weeks.
| Marker | What it’s watching for |
|---|---|
| hs-CRP (High-Sensitivity C-Reactive Protein) | Baseline inflammation — the thing you're claiming to reduce |
| Complete Blood Count (CBC) with Differential | Infection, anemia and platelet count before anything injectable |
| Comprehensive Metabolic Panel (CMP) | Liver and kidney baseline |
| Vitamin D (25-Hydroxy) | Low D slows soft-tissue and bone healing measurably |
The Inflammation Deep Dive panel covers these in one order — 10 markers, $248.35 with the discount applied.
Check results you already have → · All 103 markers A–Z
Cyclobenzaprine — frequently asked questions
What is Cyclobenzaprine?
Cyclobenzaprine (Flexeril) is a healing & recovery research compound. Structurally a tricyclic; acts centrally at the brainstem rather than on muscle itself. It doesn't relax the muscle directly — it reduces the tonic descending drive that keeps it spasming.
Is the full Cyclobenzaprine protocol on this page?
The reported research dose is on this page, along with how Cyclobenzaprine works and the evidence behind it. The protocol — how to work up to it, frequency, cycle length, time off, what not to stack it with and Coach Cam's notes — is inside Skool.
What is the half-life of Cyclobenzaprine?
Cyclobenzaprine has an approximate half-life of ~18 hours, which is part of what determines how often it's dosed.
What's the evidence behind Cyclobenzaprine?
Current evidence level: Trial-backed for acute muscle spasm; the effect fades after the first week or two. Cyclobenzaprine is offered for research purposes only and is not an approved medicine.
What Cyclobenzaprine is used for
Cyclobenzaprine appears under 2 goals in the goal router.
Related Healing & Recovery compounds
Where this goes next
The pages here are the frameworks. The protocols — the dosing, the order to correct things in, the week-by-week schedule and what to retest — are inside Skool.