Key takeaways:
Cyclobenzaprine (Amrix) is a prescription medication that treats short-term muscle spasms. It should only be used for 2 to 3 weeks at a time.
Even when used for only a short period of time, cyclobenzaprine can cause side effects. Common cyclobenzaprine side effects include drowsiness, dry mouth, and headaches. Some people may also experience an upset stomach or trouble focusing.
In rare cases, heart rhythm changes and serotonin syndrome have been reported. Talk to your healthcare team if you experience signs or symptoms of either of these concerns.
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If you’ve ever strained a muscle, you know how painful and uncomfortable it can be. Even simple activities and movements can be difficult. To help ease the pain, a healthcare professional may recommend remedies such as rest and ice, an Active Release Technique session, or rehabilitation exercises.
When needed, certain medications can also provide relief from muscle pain. Over-the-counter (OTC) anti-inflammatory medications and prescription-only muscle relaxants are two common choices. Generally, OTC medications are the first-choice option, and muscle relaxants are used only if further relief is needed. These medications are meant to act as temporary sources of relief as your muscles heal.
Cyclobenzaprine (Amrix) is one of the more common muscle relaxants. It acts in the brain and nervous system to reduce pain and tenderness and improve mobility. This oral pill is FDA approved to relieve muscle spasms that are associated with a short-term muscle injury or pain. So it should only be taken as needed, and for only 2 to 3 weeks at a time.
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Although cyclobenzaprine is a low-cost and effective treatment option, it can sometimes cause side effects that limit its use.
Cyclobenzaprine side effects at a glance
Common or mild cyclobenzaprine side effects include:
Drowsiness
Fatigue
Trouble focusing
Irritability
Weakness
Dry mouth
Headache
Upset stomach
Indigestion
Constipation
Diarrhea
Rare but serious cyclobenzaprine risks include:
Heart rhythm changes
Seizures
Serotonin syndrome
Allergic reactions
Here’s more information about seven of the top cyclobenzaprine side effects to know about.
Keep in mind: This article focuses on cyclobenzaprine’s side effects when it’s used for muscle spasms. A sublingual (under-the-tongue) form called Tonmya is also approved to treat fibromyalgia.
1. Drowsiness and fatigue
Drowsiness and fatigue are two of the most common side effects of cyclobenzaprine. Taking cyclobenzaprine during the day can cause excessive sleepiness, which can limit your ability to be productive, run errands, and perform daily tasks.
Standard cyclobenzaprine comes in three doses:
5 mg
7.5 mg
10 mg
For most people, the standard dose is 5 mg, which is taken 3 times daily as needed. The 10 mg dose is more likely to cause drowsiness than the lower doses.
Cyclobenzaprine also comes as an extended-release (ER) capsule in 15 mg and 30 mg doses. These versions are usually taken once daily, and drowsiness and fatigue rates are similar between the two doses.
If you’re feeling drowsy while taking cyclobenzaprine, talk with your prescriber about potentially lowering your dose. Another option is to take the medication at night instead of during the day to lower daytime drowsiness.
Tip: Taking cyclobenzaprine with alcohol or sedative medications like opioids can worsen drowsiness even more. It’s best to avoid these combinations.
2. Dry mouth
Cyclobenzaprine can cause dry mouth, especially in older adults. Dry mouth can make it hard to chew or swallow.
If you have a problem with dry mouth while taking cyclobenzaprine, make sure to hydrate well and consistently. Keep in mind that caffeinated or sweetened drinks aren’t generally good sources of hydration and don’t count toward these numbers.
Some other tips to help manage dry mouth include:
Sucking on ice chips
Maintaining good oral hygiene
Chewing sugar-free gum
Trying OTC saliva substitutes, which are available as gels, sprays, or lozenges
3. Headache
Headaches have been reported as a somewhat common side effect of cyclobenzaprine. Headaches may not need medical attention, but it’s important to note that they could happen.
Headaches tend to go away as your body adjusts to the medication. But you can also take an OTC pain medication, such as acetaminophen (Tylenol), with your healthcare team’s OK.
4. Upset stomach
Cyclobenzaprine can cause an upset stomach or nausea. This is not as common as some of the other listed side effects. Still, to prevent gut-related issues, it’s a good idea to take cyclobenzaprine with food.
If you do experience this side effect, it’s a good idea to avoid spicy, fried, and high-fiber foods. Sticking to bland foods, such as toast or bananas, can also help. Taking an OTC medication — such as Pepto-Bismol (bismuth subsalicylate), Mylanta (aluminum hydroxide/magnesium hydroxide/simethicone), or Tums (calcium carbonate) — is another way to soothe an upset stomach.
5. Trouble focusing
As mentioned above, cyclobenzaprine can make you drowsy. This can make it harder to focus and be alert. When you first start taking cyclobenzaprine, you may need to avoid certain tasks that require alertness until you learn how the medication affects you. It’s especially important to avoid driving and doing similar attention-oriented tasks while you’re adjusting to cyclobenzaprine.
Certain people are more likely to experience focus changes than others. This side effect can be more severe if you take cyclobenzaprine with alcohol or other medications that also cause drowsiness. Older adults and people taking doses above 5 mg might have trouble focusing every time they take cyclobenzaprine.
6. Heart rhythm changes
Heart rhythm changes are a rare but serious cyclobenzaprine side effect. These cardiac-related issues have been reported to occur in less than 1% of people taking the 10 mg dose in clinical trials.
Older adults may be more at risk for heart rhythm changes. People ages 65 years and older generally shouldn’t start cyclobenzaprine at a dose higher than 5 mg. And it’s not recommended to be used at all in people with certain pre-existing heart conditions.
If you feel or detect an irregular heartbeat, become very lightheaded, or suddenly become short of breath while taking cyclobenzaprine, call 911 or go to the ER right away. You may need additional tests to see if your heart rhythm has been affected.
7. Serotonin syndrome
Serotonin syndrome — when there is too much of the chemical serotonin in the brain — is another rare but potentially serious side effect linked to cyclobenzaprine. It doesn’t usually develop from cyclobenzaprine alone; the drug itself has a small effect on serotonin. But taking cyclobenzaprine with other medications that affect serotonin can raise the risk of serotonin syndrome. This includes medications such as:
Selective serotonin reuptake inhibitors (SSRIs)
Serotonin and norepinephrine reuptake inhibitors (SNRIs)
Triptan migraine medications
Symptoms of mild serotonin syndrome can include a fast heart rate, sweating, and trouble sleeping. More severe symptoms can include confusion, fever, and even agitation. If you feel these symptoms, it’s important to alert your healthcare team right away and ask about next steps.
Frequently asked questions
Cyclobenzaprine is an effective treatment for muscle spasms. Unlike some pain medications, it isn’t classified as a “strong” or “weak” muscle relaxant.
It’s generally safe to take cyclobenzaprine every day for a short period of time — usually up to 2 to 3 weeks. This tends to be the case as long as you don’t exceed the recommended daily dosage set by your prescriber.
Taking cyclobenzaprine every day for longer than 2 to 3 weeks isn’t recommended, though. There’s limited evidence that it helps long term, and the risk of side effects goes up the longer you take it. If you still feel like you need cyclobenzaprine daily after a few weeks, talk with your prescriber before doing so.
Taking a little more cyclobenzaprine than prescribed, such as an extra dose by mistake, may cause more drowsiness, dry mouth, or a fast heartbeat. Many people remain stable and can often be monitored at home after checking in with their healthcare team.
Taking much more than prescribed, or taking an excessive amount on purpose, is more serious. It can lead to severe confusion, agitation, or hallucinations, along with dangerous heart rhythm changes or seizures. The risk is even higher if cyclobenzaprine is paired with alcohol, opioids, benzodiazepines, or other substances that cause drowsiness. If you or someone else have overdosed, call 911 or get emergency help right away.
Most mild cyclobenzaprine side effects resolve as your body adjusts to the medication. But if side effects persist or worsen, make sure to speak with a healthcare professional. They may adjust your cyclobenzaprine dosage or recommend taking another medication altogether.
In certain situations, it’s important to seek medical care right away. Visit the nearest ER if you have any of these serious symptoms after taking cyclobenzaprine:
Seizures
Irregular heartbeat
Severe drowsiness
Cyclobenzaprine is an effective treatment for muscle spasms. Unlike some pain medications, it isn’t classified as a “strong” or “weak” muscle relaxant.
It’s generally safe to take cyclobenzaprine every day for a short period of time — usually up to 2 to 3 weeks. This tends to be the case as long as you don’t exceed the recommended daily dosage set by your prescriber.
Taking cyclobenzaprine every day for longer than 2 to 3 weeks isn’t recommended, though. There’s limited evidence that it helps long term, and the risk of side effects goes up the longer you take it. If you still feel like you need cyclobenzaprine daily after a few weeks, talk with your prescriber before doing so.
Taking a little more cyclobenzaprine than prescribed, such as an extra dose by mistake, may cause more drowsiness, dry mouth, or a fast heartbeat. Many people remain stable and can often be monitored at home after checking in with their healthcare team.
Taking much more than prescribed, or taking an excessive amount on purpose, is more serious. It can lead to severe confusion, agitation, or hallucinations, along with dangerous heart rhythm changes or seizures. The risk is even higher if cyclobenzaprine is paired with alcohol, opioids, benzodiazepines, or other substances that cause drowsiness. If you or someone else have overdosed, call 911 or get emergency help right away.
Most mild cyclobenzaprine side effects resolve as your body adjusts to the medication. But if side effects persist or worsen, make sure to speak with a healthcare professional. They may adjust your cyclobenzaprine dosage or recommend taking another medication altogether.
In certain situations, it’s important to seek medical care right away. Visit the nearest ER if you have any of these serious symptoms after taking cyclobenzaprine:
Seizures
Irregular heartbeat
Severe drowsiness
The bottom line
Cyclobenzaprine (Amrix) side effects most often include drowsiness, dry mouth, and headaches. An upset stomach and trouble focusing are also common complaints. In rare cases, this medication can cause serious concerns related to heart rhythm problems and serotonin syndrome. If you experience any cyclobenzaprine side effects like these, talk to your healthcare team about how to best address them.
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References
Cephalon. (2024). Amrix [package insert]. DailyMed.
Khan, I., et al. (2022). Cyclobenzaprine. StatPearls.
National Institute of Dental and Craniofacial Research. (2024). Dry mouth.
Preferred Pharmaceuticals. (2025). Cyclobenzaprine hydrochloride [package insert].
