Cyclobenzaprine Side Effects: What to Expect and When to Call Your Doctor

What You Should Know About Cyclobenzaprine Side Effects

Cyclobenzaprine is one of the most widely prescribed muscle relaxants in the United States, and for good reason — it's effective at relieving muscle spasms and pain. But like any medication, it comes with side effects that you should understand before you start taking it.

Most side effects are mild and go away on their own. Some, however, need medical attention right away. This guide covers what to expect, who's most at risk, and exactly when to contact your doctor.

What Is Cyclobenzaprine?

Cyclobenzaprine is a centrally acting muscle relaxant that's FDA-approved to treat muscle spasms associated with acute musculoskeletal conditions. It's typically used alongside rest and physical therapy for 2 to 3 weeks. You may know it by former brand names like Flexeril (discontinued) or the extended-release version Amrix.

It's structurally related to tricyclic antidepressants like Amitriptyline, which explains some of its side effects. For a full overview, see our guide on what is Cyclobenzaprine.

Common Side Effects

The following side effects are the most frequently reported in clinical studies and FDA labeling. They're usually mild and tend to improve as your body adjusts to the medication:

• Drowsiness or sedation — This is the most common side effect, affecting up to 39% of patients. It's most noticeable when you first start taking Cyclobenzaprine or when your dose is increased.
• Dry mouth — Reported in up to 32% of patients. Sipping water throughout the day and chewing sugar-free gum can help.
• Dizziness — Occurs in up to 11% of patients. Avoid driving or operating heavy machinery until you know how the medication affects you.
• Fatigue — Feeling more tired than usual is common, especially in the first few days.
• Constipation — The medication's anticholinergic properties can slow digestion.
• Nausea — Some people experience mild stomach upset.
• Headache — Reported occasionally.
• Blurred vision — Related to the drug's anticholinergic effects.
• Unpleasant taste — Some patients notice a metallic or bitter taste.

Most of these effects are dose-related — meaning they're more likely at higher doses (like 10 mg three times daily) compared to lower doses (5 mg three times daily).

Serious Side Effects

While rare, the following side effects are serious and require immediate medical attention:

Serotonin Syndrome

If you take Cyclobenzaprine with other serotonergic drugs — like SSRIs (Fluoxetine, Sertraline), SNRIs (Venlafaxine, Duloxetine), Tramadol, or triptans — you may be at risk for serotonin syndrome. Symptoms include:

• Agitation or restlessness
• Rapid heartbeat
• High body temperature
• Muscle twitching or rigidity
• Loss of coordination
• Diarrhea

This is a medical emergency. Call 911 or go to the emergency room immediately if you experience these symptoms. Learn more in our guide on Cyclobenzaprine drug interactions.

Cardiac Arrhythmias

Because Cyclobenzaprine is structurally similar to tricyclic antidepressants, it can affect heart rhythm. People with pre-existing heart conditions, arrhythmias, heart block, or heart failure should not take Cyclobenzaprine. Symptoms of a heart rhythm problem include:

• Rapid or irregular heartbeat
• Chest pain
• Feeling faint or lightheaded
• Shortness of breath

Seizures

Seizures are rare but have been reported, particularly in patients taking high doses or those with a history of seizure disorders.

Severe Allergic Reactions

In rare cases, Cyclobenzaprine can cause anaphylaxis or angioedema. Seek emergency help if you experience:

• Swelling of the face, lips, tongue, or throat
• Difficulty breathing
• Hives or widespread rash

Neuroleptic Malignant Syndrome-Like Reactions

Very rarely, Cyclobenzaprine has been associated with symptoms resembling neuroleptic malignant syndrome, including high fever, muscle rigidity, and altered mental status.

Who's at Higher Risk for Side Effects?

Certain groups are more likely to experience side effects from Cyclobenzaprine:

Older Adults

Cyclobenzaprine is listed on the Beers Criteria, which identifies medications that are potentially inappropriate for older adults. Seniors are more sensitive to its sedating and anticholinergic effects, which can increase the risk of falls, confusion, and urinary retention. If you're over 65, your doctor may choose a different muscle relaxant or prescribe a lower dose.

People with Liver Problems

Cyclobenzaprine is processed by the liver. If you have moderate-to-severe liver impairment, the drug may build up in your system, increasing side effects. Your doctor should start you at the lowest dose (5 mg) and monitor you closely.

Pregnant or Breastfeeding Women

Cyclobenzaprine is classified as Pregnancy Category B — animal studies haven't shown risk, but there's limited human data. It should only be used during pregnancy if clearly needed. It's unknown whether Cyclobenzaprine passes into breast milk, so breastfeeding women should talk to their doctor.

Children

Cyclobenzaprine is not recommended for children under 15 years of age.

How to Manage Common Side Effects

If you're experiencing mild side effects, here are some practical tips:

• Drowsiness: Take your dose at bedtime if your doctor approves. Avoid alcohol, which makes sedation worse. Don't drive until you know how Cyclobenzaprine affects you.
• Dry mouth: Drink plenty of water, use sugar-free gum or lozenges, and consider an over-the-counter dry mouth rinse.
• Dizziness: Get up slowly from sitting or lying positions. Avoid sudden movements.
• Constipation: Increase your fiber intake, drink more water, and stay physically active. A mild over-the-counter laxative may help.
• Nausea: Take your medication with food if stomach upset is an issue.
• Blurred vision: This usually improves as your body adjusts. Avoid tasks requiring sharp vision until it resolves.

If side effects are persistent or bothersome, talk to your doctor. They may lower your dose or suggest an alternative muscle relaxant with a different side effect profile.

When to Call Your Doctor

Contact your doctor promptly if you experience:

• Side effects that don't improve after a few days
• Side effects that significantly interfere with your daily life
• Rapid or irregular heartbeat
• Signs of an allergic reaction (rash, swelling, difficulty breathing)
• Signs of serotonin syndrome (agitation, rapid heartbeat, high temperature, muscle twitching)
• Seizures
• Severe constipation or urinary retention

Call 911 immediately if you have symptoms of serotonin syndrome, a severe allergic reaction, or a seizure.

Final Thoughts

Cyclobenzaprine is generally well-tolerated for short-term use, and most side effects are mild — drowsiness, dry mouth, and dizziness being the most common. Serious reactions are rare but important to recognize.

The key is knowing what to watch for and communicating openly with your doctor about any medications you take. If side effects are bothering you, don't stop taking Cyclobenzaprine on your own — talk to your provider about adjusting your dose or trying something else.

For more information, check out our guides on how Cyclobenzaprine works and how to save money on your prescription. And if you're having trouble finding it at your pharmacy, use Medfinder to check stock near you.