How long does it take for Flexeril to work?

Medically reviewed by Leigh Ann Anderson, PharmD. Last updated on April 28, 2025.

Official Answer by Drugs.com

Flexeril (cyclobenzaprine) is a fast-acting medicine and you should start to feel some muscle relaxant effects within 30 minutes to one hour. The drowsy side effect will set in quickly, too, so be sure you are not driving or doing other hazardous activities.

Flexeril’s duration of action is 4 to 6 hours and it is typically given 3 times per day. It may take up to 2 days for the full onset of action to begin and up to 7 days for a full effect on your muscle spasm pain.
The use of Flexeril for periods longer than two or three weeks is not typically recommended because acute muscle spasm pain usually resolves within this timeframe.
Drowsiness, fatigue and sedation can occur in up to 40% of patients and are the most common side effects. Do not drive or drink alcohol while taking this medication and always have your pharmacist or doctor check for drug interactions.

The extended-release cyclobenzaprine capsule (brand name: Amrix) is given once per day but it may take longer for you to notice its effect. One study has shown it can take up to 4 days to notice the full effect.

Drowsiness and dizziness can also occur with Amrix. Amrix should NOT be used in the elderly or in patients with impaired liver function.

If you’ve been prescribed Flexeril, chances are that you've pulled your lower back, strained your neck or twisted some other distant muscle in your body. If you sit at a desk on a daily basis, you may be all too familiar with this pain. Acute muscle injuries often result in a painful, constant spasm and interrupt work, daily activities, and especially sleep.

Speak with your healthcare provider, who may recommend you also use other methods, such as heat, ice, exercise, or other medications, such as NSAIDs or acetaminophen, for relief of muscle spasm pain.

Learn more: How much Flexeril (cyclobenzaprine) can I take?

This is not all the information you need to know about Flexeril (cyclobenzaprine) for safe and effective use and does not take the place of your doctor’s directions. Review the full product information and discuss this information and any questions you have with your doctor or other health care provider.

Is Flexeril addictive, will it show up on a drug test?

Flexeril is not a physically addictive drug like an opioid or a benzodiazepine, and is not a controlled substance. Flexeril would not normally be tested for in a routine workplace drug screen; however, there have been reports of Flexeril causing a false-positive result for tricyclic antidepressants (TCAs). Flexeril is closely related structurally to the tricyclic antidepressants, e.g., amitriptyline and imipramine. Continue reading

Can ibuprofen cause constipation?

Yes, ibuprofen may cause constipation in 1% to 10% of patients, but it is more likely to cause other stomach side effects like nausea (57%), vomiting (22%), flatulence / gas (16%), diarrhea (10%) and stomach pain or heartburn (1% to 10%). Continue reading

Can you take ibuprofen on an empty stomach?

Despite popular belief that you have to take ibuprofen with food, you can take ibuprofen on an empty stomach and this will give you faster pain relief than taking it with food. Food increases the time it takes for ibuprofen to be absorbed, although it won't affect how much is absorbed. There is actually no evidence that taking ibuprofen with food prevents gastric irritation – ibuprofen at OTC doses has a low incidence of gastric irritation anyway. Continue reading

How long does it take for Flexeril to work?

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