How Long Does Flexeril (Cyclobenzaprine) Last?

Medically Reviewed by Johnelle Smith, M.D. on

Cyclobenzaprine hydrochloride is a short-term muscle relaxer and pain medication suited to people with musculoskeletal injuries and similar medical conditions. Before starting Flexeril, it’s important to know how long it lasts and what to expect.

How Long Does Flexeril (Cyclobenzaprine) Last?

Cyclobenzaprine is a prescription muscle relaxer that provides four to six hours of relief for people with musculoskeletal conditions, including muscle spasms, sprains, and back pain.

Flexeril was the most common form of cyclobenzaprine prescribed, but it is no longer sold in the U.S. In its place are the brand names Fexmid (immediate-release) and Amrix (extended-release).

Cyclobenzaprine abuse is not a common occurrence, but it does happen, especially in cases of polysubstance abuse.

Understanding how long drugs last can help people safely take medications and reduce their risks of combining harmful medications or overdosing.

How Does Cyclobenzaprine Work?

Cyclobenzaprine belongs to a class of drugs known as skeletal muscle relaxants. The drug interacts with the central nervous system (CNS) to depress muscular hyperactivity.

With that said, cyclobenzaprine is also structurally similar to the tricyclic antidepressants that affect serotonin levels to treat depression, neuropathic pain, and attention-deficit/hyperactivity disorder (ADHD).

Due to some of these similarities, cyclobenzaprine has off-label indications for fibromyalgia and post-traumatic stress disorder (PTSD).

In any case, cyclobenzaprine is only designed for short-term use. The standard medical advice is that anything beyond a couple of weeks should be addressed with physical therapy.

Is Cyclobenzaprine Addictive?

Cyclobenzaprine is not a controlled substance like heroin or cocaine, but there are indications that abuse should be more of a concern.

Between 2004 and 2009, emergency room visits involving the non-pharmaceutical use of muscle relaxers doubled to over 50,000 per year.

While cyclobenzaprine may not share the exact same characteristics as other addictive substances, the drug’s ability to induce feelings of relaxation can create a sense of euphoria.

Signs Of Drug Abuse

Since cyclobenzaprine is not a controlled substance, people may not realize that it can still cause physical dependence or addiction.

If taken longer than the recommended time, people can start to build a tolerance and need higher doses to feel the same effects.

Some signs of Flexeril abuse include:

  • taking higher or more frequent doses than prescribed
  • mixing cyclobenzaprine with other substances
  • skipping school, work, or other obligations due to cyclobenzaprine use
  • having intense urges or cravings for cyclobenzaprine
  • continuing to use cyclobenzaprine despite negative consequences
  • trying and failing to quit cyclobenzaprine use

How Long Does Cyclobenzaprine Last?

Since the discontinuation of Flexeril, prescription cyclobenzaprine is prescribed in two forms. Given their different mechanisms, each has its own period of efficacy and detection.

These windows may be affected by a number of factors including the patient’s weight, age, sex, and general health.

Fexmid (Immediate Release)

The initial effects last between four and six hours, and it stays in the body for up to four days. The standard dose starts at 5 milligrams (mg) three times per day.

Amrix (Extended-Release)

Amrix is similar to a dose of Flexeril. The initial effects last up to 24 hours, and it stays in the body for five or six days. It is often prescribed in once-a-day 15 mg or 30 mg doses.

Detection Times For Cyclobenzaprine

How long Flexeril stays in your system depends on its half-life, which is the measure of how long it takes your body to metabolize half of a particular substance.

The half-life of cyclobenzaprine can be as short as eight hours and as long as 37 hours. On average, it’s about 18 hours for most people.

The extended-release formula does not increase the natural half-life of the drug, but it does continue to introduce the drug to your system throughout the day.

As a result, the extended-release may be detectable one or two days longer than the immediate-release formula.

Does Flexeril Show Up On Drug Tests?

Most five-panel drug tests do not screen for drugs like cyclobenzaprine. However, 10- and 12-panel drug tests can pick up on the metabolites left behind when the drug is processed.

Detection times in drug tests are as follows:

  • urine tests: detectable up to eight days after the last dose
  • blood tests: detectable up to three days after the last dose
  • hair tests: detectable up to 90 days after the last dose
  • saliva tests: detectable for less than two days, if at all

How Long Do Flexeril Side Effects Last?

There are a few common side effects associated with cyclobenzaprine, but severe or life-threatening ones are rare.

If you experience severe side effects, you should immediately call your doctor and cease using Flexeril.

Side effects typically start not long after ingesting the dose of medication, but they usually lessen as the medication works its way out of the body’s systems.

Standard Flexeril side effects are:

  • drowsiness
  • dry mouth
  • constipation
  • nausea
  • headaches
  • dizziness

Serious side effects of Flexeril include:

  • fast or irregular heartbeat
  • chest pain
  • hallucinations
  • numbness and weakness
  • slurred speech

How Long Does Detox Take?

Suddenly stopping cyclobenzaprine after an extended period of substance use can lead to withdrawal. Fortunately, the symptoms are mild, so Flexeril detox may be safe at home.

For the most part, people describe flu- or cold-like withdrawal symptoms such as aches and pains, headaches, and nausea.

The exact length of time it takes to detox can depend on factors like the length and frequency of Flexeril use or size dosage, but it typically takes a few days to a week.

Flexeril Use FAQs

If you need more information, we’ve gathered some of the most common questions and answers about Flexeril below.

Flexeril overdoses are uncommon. However, the risk is significantly increased by mixing cyclobenzaprine with other CNS depressants like alcohol, benzodiazepines, and antidepressants.

The combined effects of two or more CNS depressants can lead to severe sedation, respiratory depression, or heart attack. If you or a loved one experience any of these symptoms, seek emergency medical assistance.

Flexeril is dangerous when mixed with other central nervous system depressants. This can cause respiratory depression, which may lead to comas or other adverse physical side effects.

It may also interact with anticonvulsants (seizure medications), vitamins, or antianxiety drugs. Be sure your healthcare provider knows about any other prescription drugs before taking Flexeril.

Flexeril and other medications containing cyclobenzaprine are not controlled substances since they are not commonly abused. Flexeril abuse usually involves other substances.

A few examples of controlled substances with higher risks of addiction include marijuana, ketamine, or Adderall.

Seeking Help For A Flexeril Addiction

If you or a loved one are battling a muscle relaxer or cyclobenzaprine addiction, you are not alone. We can help you find addiction treatment options to get you on the path to recovery.

Whether you need guided inpatient care or a more hands-off outpatient approach, we will be able to find the program that fits you.

To learn more, get in touch with one of our treatment specialists via our helpline. It only takes one phone call to turn things around.

This page does not provide medical advice. See more

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Medically Reviewed by
Johnelle Smith, M.D. on
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