Generic name: cyclobenzaprine [ sye-kloe-BEN-za-preen ]
Brand names:   Tonmya, Amrix, Comfort Pac with Cyclobenzaprine, Fexmid, Flexeril
Drug class: Skeletal muscle relaxants

Medically reviewed by Melisa Puckey, BPharm. Last updated on Dec 4, 2025.

What is cyclobenzaprine?

Cyclobenzaprine is a prescription muscle relaxant used for muscle spasms caused by sprains and strains, including acute low back or neck pain. Cyclobenzaprine is used short-term, together with rest and physical therapy, when used to treat skeletal muscle conditions such as pain or injury.

Cyclobenzaprine is also used for fibromyalgia to help reduce pain, improve sleep quality, and relieve fatigue. For this condition, it is available as a sublingual tablet under the brand name Tonmya. Tonmya may begin to relieve pain as early as the first week of treatment, with continued improvements in both pain and sleep over time.

Cyclobenzaprine works by blocking nerve impulses (or pain sensations) that are sent to your brain. Cyclobenzaprine acts primarily through the brain stem rather than the spinal cord and does not act directly on skeletal muscle, and will not reduce muscle spasms due to central nervous system (CNS) disease, such as cerebral palsy.

Warnings

You should not use cyclobenzaprine if you have an allergy to the medication, a certain type of thyroid disorder (hyperthyroidism), heart block, congestive heart failure, a heart rhythm disorder, or you have recently had a heart attack.

Do not use cyclobenzaprine if you have taken an MAO inhibitor in the past 14 days, such as isocarboxazid, linezolid, phenelzine, rasagiline, selegiline, or tranylcypromine.

Before taking this medicine

You should not use cyclobenzaprine if you are allergic to it, or if you have:

• hyperthyroidism;

• heart block, heart rhythm disorder, congestive heart failure; or

• if you have recently had a heart attack.

The safety and effectiveness of cyclobenzaprine HCl for muscle spasm in patients below 15 years of age have not been established. Cyclobenzaprine for fibromyalgia (Tonmya) is only approved for use in adults.

Do not use cyclobenzaprine if you have taken an MAO inhibitor in the past 14 days. A dangerous drug interaction could occur. MAO inhibitors include isocarboxazid, linezolid, phenelzine, rasagiline, selegiline, and tranylcypromine.

Some medicines can interact with cyclobenzaprine and cause a serious condition called serotonin syndrome. Be sure your doctor knows if you also take stimulant medicine, opioid medicine, herbal products, or medicine for depression, mental illness, Parkinson's disease, migraine headaches, serious infections, or prevention of nausea and vomiting. Ask your doctor before making any changes in how or when you take your medications.

To make sure cyclobenzaprine is safe for you, tell your doctor if you have:

• thyroid disease;

• liver disease;

• glaucoma;

• enlarged prostate; or

• problems with urination.

Use in the Elderly

In older adults, cyclobenzaprine can build up to higher levels in the body and is more likely to cause side effects such as confusion, hallucinations, heart problems and falls. Therefore cyclobenzaprine should only be used in the elderly if clearly needed, and treatment is usually started at a low dose (5 mg) and increased slowly under medical supervision.

Pregnancy

Cyclobenzaprine may cause neural tube defects when used two weeks before conception and through the first trimester of pregnancy. Because neural tube development occurs early in pregnancy, often before pregnancy is recognized, females of reproductive potential should be made aware of the potential risk to the fetus and should avoid use of this medicine two weeks prior to conception and through the first trimester of pregnancy. Females of reproductive potential should be advised to use effective contraception during cyclobenzaprine treatment and for two weeks after the final dose. You may be required to do a pregnancy test before starting this medicine to confirm that you are not pregnant.

Breastfeeding

Tell your healthcare provider if you are breastfeeding or plan to breastfeed while using cyclobenzaprine. It is not known whether this cyclobenzaprine passes into your breast milk. Talk to your healthcare provider about the best way to feed your baby while using this medicine

How should I take cyclobenzaprine?

Cyclobenzaprine dose for muscle spasm 

Cyclobenzaprine is usually taken for up to 2 or 3 weeks. Follow all directions on your prescription label and read all medication guides or instruction sheets. Your doctor may occasionally change your dose.

Follow your doctor's dosing instructions very carefully.

Swallow the capsule whole and do not crush, chew, break, or open it. Call your doctor if your symptoms do not improve after 3 weeks, or if they get worse.

Cyclobenzaprine for fibromyalgia

The recommended starting dose of Tonmya is 1 sublingual tablet once daily at bedtime for the first 2 weeks, followed by 2 sublingual tablets once daily at bedtime thereafter.

Recommended Tonmya Dosage

• Initial dose (Days 1–14): 2.8 mg (1 sublingual tablet) once daily at bedtime.
• Maintenance dose (Day 15 and onward): 5.6 mg (2 sublingual tablets) once daily at bedtime.
• Maximum recommended dose: 5.6 mg once daily.
• Dose modification: the recommended maximum dose in geriatric patients and patients with mild hepatic impairment is 2.8 mg  (1 sublingual tablet) once daily at bedtime.
• Tonmya is not recommended in patients with moderate or severe hepatic impairment.

Tonmya sublingual tablets: 2.8 mg of cyclobenzaprine hydrochloride

What happens if I miss a dose?

Take the medicine as soon as you can, but skip the missed dose if it is almost time for your next dose. Do not take two doses at one time.

What happens if I overdose?

Seek emergency medical attention or call the Poison Help line at 1-800-222-1222. An overdose of cyclobenzaprine can be fatal.

Overdose symptoms may include severe drowsiness, vomiting, fast heartbeats, tremors, agitation, or hallucinations.

What to avoid

Avoid driving or hazardous activity until you know how this medicine will affect you. Your reactions could be impaired.

Avoid drinking alcohol. Dangerous side effects could occur.

Cyclobenzaprine side effects

Get emergency medical help if you have signs of an allergic reaction to cyclobenzaprine: hives; difficult breathing; swelling of your face, lips, tongue, or throat.

Stop using this medicine and call your doctor at once if you have:

• fast or irregular heartbeats;

• chest pain or pressure, pain spreading to your jaw or shoulder; or

• sudden numbness or weakness (especially on one side of the body), slurred speech, balance problems.

Seek medical attention right away if you have symptoms of serotonin syndrome, such as: agitation, hallucinations, fever, sweating, shivering, fast heart rate, muscle stiffness, twitching, loss of coordination, nausea, vomiting, or diarrhea.

Serious side effects may be more likely in older adults.

Common cyclobenzaprine side effects may include:

• drowsiness, tiredness;

• headache, dizziness;

• dry mouth; or

• upset stomach, nausea, constipation.

This is not a complete list of side effects and others may occur. Call your doctor for medical advice about side effects. You may report side effects to FDA at 1-800-FDA-1088.

Related/similar drugs

What other drugs will affect cyclobenzaprine?

Using cyclobenzaprine with other drugs that make you drowsy can worsen this effect. Ask your doctor before using opioid medication, a sleeping pill, a muscle relaxer, or medicine for anxiety or seizures.

Tell your doctor about all your other medicines, especially:

• MAO inhibitors, such as isocarboxazid, linezolid, phenelzine, rasagiline, selegiline, or tranylcypromine;

• Any antidepressant or anxiety medications;

• bupropion (Zyban, for smoking cessation or Wellbutrin, for depression);

• meperidine;

• tramadol;

• verapamil;

• cold or allergy medicine that contains an antihistamine (Benadryl and others);

• medicine to treat Parkinson's disease;

• medicine to treat excess stomach acid, stomach ulcer, motion sickness, or irritable bowel syndrome;

• medicine to treat overactive bladder; or

• bronchodilator asthma medication.

This list is not complete. Other drugs may interact with cyclobenzaprine, including prescription and over-the-counter medicines, vitamins, and herbal products. Not all possible drug interactions are listed here.

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Further information

Remember, keep this and all other medicines out of the reach of children, never share your medicines with others, and use cyclobenzaprine only for the indication prescribed.

Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.

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