Muscle Relaxers Compared: Cyclobenzaprine, Tizanidine, Baclofen and Carisoprodol

Four drugs get called muscle relaxants in the same sentence every day, and they are not interchangeable. Cyclobenzaprine and carisoprodol treat short lived spasm after a back strain. Baclofen and tizanidine treat spasticity from multiple sclerosis and spinal cord injury. The two groups have different targets, different dosing ceilings, different sedation profiles and different rules about how long you can stay on them. This guide puts the four side by side using the numbers that come from placebo controlled trials rather than general impressions.

Two different problems share one label

The class name skeletal muscle relaxant covers medicines used for two conditions that have little in common. The first is muscle spasm from a peripheral musculoskeletal problem, meaning a strained back, a wrenched neck or an acute injury. The spasm is painful, self limiting and usually resolves inside a couple of weeks. The second is spasticity, a velocity dependent increase in muscle tone caused by damage to upper motor neurons, seen in multiple sclerosis and spinal cord disease. Spasticity does not resolve on its own.

Drugs aimed at the first problem are usually described as antispasmodics. Cyclobenzaprine and carisoprodol sit here. Drugs aimed at the second are antispastics, and baclofen sits squarely there. Tizanidine is the one agent with a foot in both camps, licensed in the United States for spasticity while being used widely off label for acute musculoskeletal pain.

This distinction is written into the labels. The cyclobenzaprine label states plainly that the drug has not been found effective for spasticity from cerebral or spinal cord disease, or in children with cerebral palsy. The baclofen label runs the same logic in reverse and states that baclofen is not indicated for skeletal muscle spasm from rheumatic disorders. Picking from the wrong half of the class is one of the most common avoidable errors in this drug group.

How each of the four actually works

Cyclobenzaprine is structurally a tricyclic, closely related to amitriptyline and imipramine. That family resemblance explains almost everything about its side effect profile, including the dry mouth, the drowsiness and the cardiac contraindications. It acts within the central nervous system rather than at the muscle itself.

Tizanidine is a central alpha 2 adrenergic agonist. It reduces spasticity by increasing presynaptic inhibition of motor neurons, with the largest effect on polysynaptic pathways. The net result is reduced facilitation of spinal motor neurons. Being an alpha 2 agonist also explains its blood pressure effects, which are not incidental.

Baclofen is an analogue of gamma aminobutyric acid and inhibits both monosynaptic and polysynaptic reflexes at spinal level, probably by hyperpolarising afferent terminals. The label is candid that a conclusive link between its GABA activity and its clinical effect has not been established. Baclofen is cleared by the kidneys largely unchanged, which matters a great deal in renal impairment.

Carisoprodol has the least well defined mechanism of the four. The label states that the mechanism has not been clearly identified, and that animal work links muscle relaxation to altered interneuronal activity in the spinal cord and the descending reticular formation. Carisoprodol is broken down in the liver by CYP2C19 into meprobamate, a sedative and a controlled substance in its own right. Much of what carisoprodol does over a full day is attributable to that metabolite.

Sedation compared using placebo controlled numbers

Sedation is the reason most people stop taking these drugs, so it deserves numbers rather than adjectives. The figures below come from placebo controlled trials in each drug label. They come from separate studies in different populations, so the raw percentages cannot be compared head to head. What can be compared, with care, is the gap between drug and placebo within each trial.

Drug and dose	Drowsiness on drug	Drowsiness on placebo	Gap over placebo
Cyclobenzaprine 5 mg three times daily	29 percent	10 percent	19 points
Cyclobenzaprine 10 mg three times daily	38 percent	10 percent	28 points
Carisoprodol 250 mg four times daily	13 percent	6 percent	7 points
Carisoprodol 350 mg four times daily	17 percent	6 percent	11 points
Baclofen, titrated dosing	63 percent	36 percent	27 points

Two things stand out. The first is that carisoprodol, the agent with the worst reputation for misuse, produced the smallest sedation gap over placebo in its own trials. The second is that cyclobenzaprine sedation is strongly dose dependent, with the gap over placebo rising from 19 points at 5 mg to 28 points at 10 mg. That single fact is the argument for starting at 5 mg rather than 10 mg, and it is covered in detail in our cyclobenzaprine dosage guide.

The baclofen figure comes from a controlled study of 175 patients with spasticity, a population already prone to fatigue, which is why the placebo rate is 36 percent rather than the 6 to 10 percent seen in back pain trials. Dry mouth follows a similar dose pattern with cyclobenzaprine, reported by 21 percent at 5 mg and 32 percent at 10 mg against 7 percent on placebo.

Real world rates run lower than trial rates

The cyclobenzaprine label carries an unusual and useful comparison. It reports adverse event rates from controlled clinical studies alongside rates from a postmarketing surveillance programme of 7,607 patients taking the same 10 mg dose.

Adverse reaction	Controlled trials, 10 mg	Surveillance programme, 10 mg
Drowsiness	39 percent	16 percent
Dry mouth	27 percent	7 percent
Dizziness	11 percent	3 percent

The gap is roughly two to four fold across all three symptoms. Trials actively question patients at every visit, while surveillance captures what people report on their own. Neither number is wrong. Trial figures set the ceiling of what to expect, and surveillance figures are closer to what most people actually notice.

Cyclobenzaprine at a glance

Licensed as an adjunct to rest and physical therapy for muscle spasm from acute painful musculoskeletal conditions. Usual dose is 5 mg three times daily, raised to 10 mg three times daily if needed. Use beyond two or three weeks is not recommended, because evidence of effectiveness past that point does not exist and acute spasm is generally short lived anyway. Older adults and anyone with hepatic impairment need less frequent dosing.

The contraindication list is longer than most people expect for a muscle relaxant, and it reflects the tricyclic structure. Cyclobenzaprine is contraindicated with monoamine oxidase inhibitors or within 14 days of stopping one, during the acute recovery phase of myocardial infarction, in arrhythmias, heart block or conduction disturbance, in congestive heart failure and in hyperthyroidism. Available strengths on this site include Flexabenz 5mg, Flexabenz 15mg, Skelebenz 15mg and the extended release Cycloboon ER 15mg.

Tizanidine at a glance

Licensed for the management of spasticity. Because the therapeutic effect is short, tizanidine is designed to be timed around the activities where relief matters most rather than taken as blanket background cover. Starting dose is 2 mg every six to eight hours, up to three doses in 24 hours, increased by 2 mg to 4 mg per dose every one to four days. Maximum total daily dose is 36 mg, and single doses above 16 mg have never been studied.

Tizanidine carries two features the other three do not. It can cause hepatocellular liver injury, so the label recommends checking aminotransferase levels at baseline and one month after the maximum dose is reached. It also caused formed visual hallucinations or delusions in 5 of 170 patients, around 3 percent, across two North American controlled studies. Most of those patients knew the events were not real, but one developed psychosis and one still had problems two weeks after stopping. Stock includes Tizan 2mg and Trinex 2mg, and the interaction detail is covered in our tizanidine dosage and interactions guide.

Baclofen at a glance

Licensed for spasticity from multiple sclerosis, particularly flexor spasms with pain, clonus and muscular rigidity, and of some value in spinal cord injury and other spinal cord disease. The label states that efficacy in stroke, cerebral palsy and Parkinson disease has not been established. Dosing is a slow titration: 5 mg three times daily for three days, then 10 mg three times daily for three days, then 15 mg, then 20 mg, with most patients settling between 40 mg and 80 mg daily and 80 mg as the ceiling.

Baclofen is excreted primarily unchanged by the kidneys, so renal impairment demands caution and often a dose reduction. A 2025 systematic review of 1,618 people with oral baclofen toxicity or withdrawal found central nervous system depression in 68 percent, seizures in 36 percent and respiratory depression in 21 percent, concentrated at doses of 300 mg and above. Around 54.5 percent needed mechanical ventilation, and 97.7 percent recovered fully. Strengths available include Liofen 5mg, Liofen 10mg, Liofen 25mg and the extended release Liofen XL 10mg.

Carisoprodol and why it is controlled

Carisoprodol is licensed for relief of discomfort from acute painful musculoskeletal conditions in adults, at 250 mg or 350 mg three times daily and at bedtime, for up to two or three weeks. The FDA approved strengths are 250 mg and 350 mg. Nothing in the label supports a single dose above 350 mg.

Two pharmacological facts drive its reputation. Carisoprodol itself has a half life of about two hours, but its CYP2C19 metabolite meprobamate has a half life of roughly ten hours and accumulates through the day. And CYP2C19 activity varies genetically. Poor metabolisers have a four fold increase in carisoprodol exposure with about half the meprobamate exposure. Poor metabolisers make up roughly 3 to 5 percent of Caucasian and African American populations and 15 to 20 percent of Asian populations, so identical dosing produces very different blood levels in different people.

The abuse and withdrawal risk is documented rather than theoretical. A published case described a man taking upwards of 30 tablets daily, at least 10,500 mg, who stopped abruptly and developed anxiety, tremors, muscle twitching, insomnia and auditory and visual hallucinations within 48 hours, peaking on day four. Carisoprodol is a Schedule IV controlled substance in the United States. In Europe the outcome was stronger: the CHMP concluded in November 2007 that the risks outweighed the benefits and recommended suspending all carisoprodol marketing authorisations, with national withdrawals following in 2008, including in the United Kingdom. Its metabolite meprobamate had its own oral marketing authorisations suspended across the EU in 2012. Pain O Soma 350mg is the strength that matches the approved label. Carisoprodol is also contraindicated in anyone with a history of acute intermittent porphyria or a hypersensitivity reaction to a carbamate such as meprobamate, a contraindication that has no equivalent among the other three drugs here.

What the comparative evidence base actually supports

The most useful comparative work is a systematic review of 101 randomised trials covering the whole class. It reached four conclusions worth repeating. Baclofen, tizanidine and dantrolene have fair evidence of benefit over placebo in spasticity, mostly in multiple sclerosis. Baclofen and tizanidine are roughly equivalent to each other for efficacy in spasticity. Overall adverse event rates between the two are similar, but the pattern differs, with tizanidine producing more dry mouth and baclofen producing more weakness. And cyclobenzaprine, carisoprodol, orphenadrine and tizanidine all have fair evidence of benefit over placebo in musculoskeletal conditions.

What the review could not find was good quality evidence that any one agent outperforms another within its own half of the class. No included trial was rated good quality, and adverse event assessment was rarely rigorous. So the honest position is that choice between these drugs is driven mostly by side effect tolerance, interactions and comorbidity rather than by demonstrated superiority.

Long term use is where the evidence runs out

Tighter opioid prescribing has pushed more people onto muscle relaxants for chronic pain, and a 2024 systematic review looked at whether that works. It pooled 30 randomised trials with 1,314 participants and 14 cohort studies with 1,168 participants, covering nine different agents. Baclofen appeared in 11 studies, tizanidine in 8 and cyclobenzaprine in 7. Most studies ran only four to six weeks.

The findings split cleanly. Long term use appeared to help people with painful spasms or cramps and with neck pain. It did not appear better than placebo for low back pain, fibromyalgia or headache. Sedation and dry mouth were the most common adverse effects throughout. The review closed with advice to consider deprescribing when pain goals are not being met, which is a reasonable checkpoint for anyone who has drifted past a few months on any of these drugs. If nerve pain rather than spasm is the real problem, a gabapentinoid is a different mechanism and our pregabalin versus gabapentin comparison covers that route.

Interactions that change which drug you can take

Tizanidine has the sharpest interaction problem of the four. Strong CYP1A2 inhibitors are contraindicated outright. Fluvoxamine raised tizanidine peak concentration 12 fold and total exposure 33 fold in a study of ten healthy subjects. Ciprofloxacin raised them 7 fold and 10 fold. Moderate and weak CYP1A2 inhibitors including amiodarone, mexiletine, propafenone, verapamil, cimetidine, famotidine, acyclovir and ticlopidine should be avoided. Combined oral contraceptives cut tizanidine clearance by half.

Cyclobenzaprine has the serotonin problem. Serotonin syndrome has been reported when it is combined with SSRIs, SNRIs, tricyclics, tramadol, bupropion, meperidine, verapamil or monoamine oxidase inhibitors. Given how commonly tramadol and an SSRI appear on the same medication list as a muscle relaxant, this is not an edge case.

Carisoprodol interacts through CYP2C19. Omeprazole and fluvoxamine increase carisoprodol exposure while lowering meprobamate exposure, and rifampicin, St John wort and low dose aspirin push it the other way. All four drugs stack additively with alcohol, benzodiazepines, opioids and tricyclic antidepressants.

Stopping: which ones need a taper

Cyclobenzaprine and carisoprodol are short course drugs, and a two to three week course generally does not need a formal taper, though carisoprodol taken at high dose for longer does. Baclofen and tizanidine both need deliberate withdrawal.

Abrupt baclofen withdrawal has caused hallucinations and seizures, so the dose should be reduced slowly unless a serious adverse reaction forces immediate cessation. Tizanidine withdrawal produces rebound hypertension, tachycardia and increased muscle tone, and the label advises reducing by 2 mg to 4 mg per day, with particular care in anyone who has been on 20 mg to 28 mg daily for nine weeks or longer or who is also taking opioids.

How the choice is usually made

For an acute back or neck strain in an otherwise well adult, cyclobenzaprine at 5 mg is the common first choice, taken for two weeks at most and often only at night because of the sedation. If the person takes an SSRI, an SNRI or tramadol, that choice becomes harder and a non serotonergic option is preferable.

For spasticity from multiple sclerosis or spinal cord injury, baclofen and tizanidine are the two realistic options and the evidence does not separate them on efficacy. Baclofen suits people who need steady all day tone reduction and have adequate kidney function. Tizanidine suits people who need targeted relief around specific activities, provided they are not on ciprofloxacin, fluvoxamine or an oral contraceptive and are willing to have liver enzymes checked. Anyone on antihypertensives needs blood pressure monitoring with tizanidine.

Carisoprodol sits last in most modern prescribing sequences, not because it fails to work but because meprobamate accumulation, genetic variability in metabolism and documented dependence make the risk to benefit balance worse than the alternatives at the same level of relief. You can view the full chronic pain treatment range for the wider set of options.

Safety information

Every drug discussed here is prescription only in the United States, United Kingdom, Australia and Canada, and carisoprodol is additionally a Schedule IV controlled substance in the United States. All four impair the mental and physical ability needed to drive or operate machinery, and those effects are additive with alcohol. None of them treats the underlying cause of pain or spasticity, and all of them work best alongside rest, physiotherapy and graded movement. Speak to a licensed clinician before starting, changing or stopping any of these medicines, particularly if you have liver disease, kidney disease, heart rhythm problems or take antidepressants.

References

• FDA prescribing information, cyclobenzaprine hydrochloride tablets: indications, dosage and administration, contraindications, warnings and adverse reactions sections
• FDA prescribing information, tizanidine hydrochloride tablets: sections 2.1 to 2.6, 5.1 to 5.6, 7.1 to 7.6 and 12.1 to 12.3
• FDA prescribing information, carisoprodol tablets: sections 5.1 to 5.3, 6.1, 7.1, 7.2 and 12.3
• DailyMed label, baclofen tablets USP: indications, dosage, warnings and adverse reactions sections
• Chou R and colleagues. Comparative efficacy and safety of skeletal muscle relaxants for spasticity and musculoskeletal conditions: a systematic review. PMID 15276195
• Long term use of muscle relaxant medications for chronic pain: a systematic review. PMID 39298168
• Clinical presentations and treatment of baclofen toxicity and withdrawal: a systematic review. PMID 41555041
• Carisoprodol withdrawal syndrome. PMID 15585447
• Pharmacotherapy for spine related pain in older adults. PMID 35754070