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

What to Know About Cyclobenzaprine Side Effects

Cyclobenzaprine is one of the most widely prescribed muscle relaxants in the United States, used to treat painful muscle spasms associated with acute musculoskeletal conditions. Like all medications, it comes with potential side effects — some mild and manageable, others that require immediate medical attention.

This guide breaks down exactly what to expect when taking Cyclobenzaprine, which side effects are normal, and when you should call your doctor.

What Is Cyclobenzaprine?

Cyclobenzaprine (formerly sold as Flexeril and available as the extended-release brand Amrix) is a centrally acting muscle relaxant that's structurally related to tricyclic antidepressants. It works in the brainstem to reduce muscle hyperactivity without acting directly on your muscles. It's typically prescribed for short-term use (2–3 weeks) alongside rest and physical therapy.

For a complete overview, see our guide on what Cyclobenzaprine is and how it's used.

Common Side Effects

Most people who take Cyclobenzaprine experience at least one side effect, especially during the first few days. These are generally mild and tend to improve as your body adjusts to the medication.

Based on FDA labeling, the most common side effects include:

• Drowsiness or sedation — Affects up to 39% of patients. This is the most frequently reported side effect and the reason Cyclobenzaprine is often taken at bedtime.
• Dry mouth — Reported in up to 32% of patients. Caused by the medication's anticholinergic properties.
• Dizziness — Affects up to 11% of patients. Usually most noticeable when standing up quickly.
• Fatigue — General tiredness beyond just drowsiness.
• Constipation — Another anticholinergic effect that may worsen with other medications that have similar properties.
• Nausea — Usually mild and often improves after the first few days.
• Headache — Can occur, especially when starting the medication.
• Blurred vision — Temporary visual changes related to anticholinergic effects.
• Unpleasant taste — Some patients report a metallic or unusual taste in the mouth.

These side effects are dose-related — meaning they're more likely at 10 mg three times daily than at 5 mg three times daily. If side effects are bothersome, your doctor may lower your dose.

Serious Side Effects

While uncommon, some side effects of Cyclobenzaprine are serious and require immediate medical attention. Call your doctor or go to the emergency room if you experience any of the following:

Serotonin Syndrome

Because Cyclobenzaprine is structurally similar to tricyclic antidepressants, it can contribute to a dangerous condition called serotonin syndrome — especially when combined with other serotonergic medications like SSRIs (Prozac, Zoloft, Lexapro), SNRIs (Cymbalta, Effexor), triptans, or Tramadol.

Symptoms of serotonin syndrome include:

• Agitation or restlessness
• Rapid heartbeat
• High blood pressure
• Dilated pupils
• Muscle twitching or rigidity
• Heavy sweating
• Diarrhea
• High fever

Serotonin syndrome is a medical emergency. If you notice these symptoms, go to the ER immediately.

Cardiac Arrhythmias

Due to its structural similarity to tricyclic antidepressants, Cyclobenzaprine can affect heart rhythm. Seek immediate care if you experience:

• Rapid, pounding, or irregular heartbeat
• Chest pain
• Fainting or near-fainting

Seizures

Though rare, seizures have been reported with Cyclobenzaprine use, particularly at higher doses or in combination with other medications that lower the seizure threshold (like Bupropion).

Allergic Reactions

Severe allergic reactions are rare but possible. Seek emergency care for:

• Swelling of the face, lips, tongue, or throat
• Difficulty breathing or swallowing
• Hives or severe skin rash

Who's at Higher Risk for Side Effects?

Certain groups of people are more likely to experience side effects or should use Cyclobenzaprine with extra caution:

Older Adults

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

People with Liver Problems

Cyclobenzaprine is processed by the liver. If you have moderate-to-severe liver disease, the medication may build up in your system, increasing the risk and severity of side effects. Liver impairment patients should start at 5 mg and use with caution.

People Taking Multiple Medications

The more medications you take — especially CNS depressants, serotonergic drugs, or anticholinergic medications — the higher your risk of serious interactions. Review our guide on Cyclobenzaprine drug interactions for a complete list.

Pregnant or Breastfeeding Women

Cyclobenzaprine is classified as Pregnancy Category B, meaning animal studies have not shown risk but there is limited human data. It's unknown whether Cyclobenzaprine passes into breast milk. Discuss the risks and benefits with your doctor if you're pregnant, planning to become pregnant, or breastfeeding.

How to Manage Common Side Effects

If you're experiencing mild side effects, these strategies can help:

For Drowsiness

• Take your dose at bedtime or in the evening when sedation won't affect your day
• Avoid driving or operating heavy machinery until you know how the medication affects you
• Don't combine with alcohol, which significantly worsens drowsiness
• Ask your doctor about starting at the lower 5 mg dose

For Dry Mouth

• Sip water throughout the day
• Chew sugar-free gum or suck on sugar-free hard candy
• Use an over-the-counter dry mouth rinse (like Biotene)
• Avoid caffeine and alcohol, which can worsen dry mouth

For Dizziness

• Stand up slowly from sitting or lying positions
• Sit on the edge of the bed for a moment before standing
• Stay hydrated
• Avoid sudden movements

For Constipation

• Increase fiber intake (fruits, vegetables, whole grains)
• Drink plenty of water
• Stay physically active as your condition allows
• Ask your pharmacist about a gentle stool softener if needed

For Nausea

• Take the medication with food
• Eat smaller, more frequent meals
• Avoid greasy or spicy foods when starting the medication

When to Call Your Doctor

Contact your doctor if:

• Side effects are interfering with your daily life and not improving after a few days
• You notice new or worsening symptoms after changing your dose
• You're experiencing symptoms of serotonin syndrome, cardiac issues, or allergic reaction (go to the ER for these)
• You feel like the medication isn't working for your muscle spasms
• You've been taking Cyclobenzaprine for more than 3 weeks and haven't discussed ongoing use with your prescriber

Never stop taking a prescribed medication without talking to your doctor first, even if you're experiencing side effects. Your doctor can adjust your dose, switch you to a different medication, or suggest ways to manage the side effects.

Final Thoughts

Cyclobenzaprine is generally well-tolerated for short-term use, but drowsiness and dry mouth are very common. Most side effects are mild and improve within a few days. Serious side effects like serotonin syndrome and cardiac arrhythmias are rare but important to recognize.

The key is to stay informed, take the medication as directed, and communicate with your doctor about any concerns. If you're looking for Cyclobenzaprine at a good price, check out our guide on coupons and savings programs. And if you're having trouble finding it in stock, Medfinder can help you locate a pharmacy near you that has it available.