How Long Does Flexeril Stay In Your System? Cyclobenzaprine Half-Life

Last Updated: April 29, 2024

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Flexeril, the brand-name version of the drug Cyclobenzaprine, is a muscle relaxant medication with an antidepressant activity used for the treatment of painful muscle spasms and other musculoskeletal conditions. Cyclobenzaprine muscle-relaxing effects are felt within about half an hour of administration, and the drug starts to be detectable in urine, blood, hair, and saliva within a few hours.

This article focuses on Flexeril half-life, how long it stays in the system, variables that can affect that, Cyclobenzaprine drug tests, and ways of treatment or how to get it out of the body safely.

How Long Does Flexeril Stay In the System?

There are many influencing factors that could affect how long Flexeril stays in the system. But for most users, and with Flexeril half-life of 18 hours considered, getting the drug out of the body will take 4 to 5 days after the last dose was taken. However, it is important to note that its elimination half-life can range from 8 to 37 hours. That said, it may just take 1.83 days for some to eliminate the drug or up to 8.48 days for others.

How Long Flexeril Stays In Urine

It is known for a fact that the effective half-life of Cyclobenzaprine is 18 hours. This means that a person with an average weight who takes the standard prescribed dose will have the drug undetectable in their urine in 4 days after its last use. Meanwhile, the drug can stay detectable for up to 8 days for people who are taking higher doses. These durations may differ depending on use habits, metabolism, and health.

How Long Flexeril Stays In The Blood

Detectability in the blood will again be based on Flexeril half-life of 18 hours, and with that, it can be assumed that traces can be detected from 3 to 4 days after the last dose. Still, remember that Cyclobenzaprine duration in the blood is highly dependent on several factors and that the window of detection can be longer.

How Long Cyclobenzaprine Stays in the Hair

Hair tests are rarely used to screen for Flexeril, but they can detect traces of Flexeril (and other drugs) for the longest period out of all tests. While most patients will eliminate it from their bodies within 72 hours, traces can show up in the hair three months or 90 days after the last intake. Drugs are deposited into the hair through the follicle, and hair growth averages at around a half-inch per month. Lastly, a hair test requires 1.5 inches of hair measured from the root for accuracy. With all that said, it would take around three months for Flexeril to be undetectable.

How Long Flexeril Stays in Saliva

Like hair tests, it is very uncommon to get a saliva test for Flexeril. There is not much information about the amount of time it is detectable in saliva, but usually, the drug only stays for up to 36 hours or 1.5 days after consumption. The detection window may, of course, extend depending on the extent of usage.

How Long Does Flexeril Stay in Your System?

Cyclobenzaprine Drug Test

For Flexeril to be detected, the testing kit would have to specifically be a Cyclobenzaprine drug test. It won’t be detected on a standard 12-panel drug test as it is not visible on regular blood, saliva, or urine tests. However, it does have a very similar structure as tricyclic antidepressants, and hence, it can be detected as an antidepressant drug and trigger a false positive. 12-panel drug tests vary in terms of what drugs they search for, though, so Cyclobenzaprine will only show up if a specific test is carried out for that. And even then, it will only show up as a TCA.

Flexeril Half-Life

18 hours is the effective half-life of Cyclobenzaprine, and it is known to be eliminated quite slowly from the body. Keep in mind, though, that Flexeril half-life can be as low as 7 hours and as high as 37 hours, depending on the formulation.

Flexeril pill in a hand.

Flexeril Mechanism of Action

As for the Cyclobenzaprine mechanism, it acts within the central nervous system and mainly in the brainstem. It acts on the noradrenergic neurons from the locus coeruleus and the serotonergic neurons from the medullary raphe. This leads to an increased release and modulation of norepinephrine through the gamma fibers that innervate and inhibit alpha motoneurons present in the ventral horn of the spinal cord. Therefore, the rate and force of muscle contraction are decreased, the muscles are relaxed, and spasms are relieved. Also, since the Cyclobenzaprine mechanism helps block the pain sensations being sent from sore muscles to the brain, one does not recognize or feel it.

Cyclobenzaprine is known to have a tricyclic structure that is contained by several therapeutic agents which are primarily metabolized by cytochromes CYP3A4 and CYP1A2 in liver microsomes.

How Long Does It Take For Cyclobenzaprine To Work?

Cyclobenzaprine comes in 5, 7.5, or 10mg immediate-release tablets and 15 or 30mg extended-release tablets. Typically, though, pain relief and its muscle-relaxing effects are noted within 20 to 30 minutes of oral administration.

How Long Does Flexeril Last?

Flexeril actively works on the muscles for four to 24 hours, depending on the formulation taken. The immediate-release tablets’ effects typically last for 4 to 6 hours, while the extended-release tablets can last up to 24 hours.

Flexeril drug test.

Variables That Affect How Long Flexeril Stays In System

Just like other drugs and medications, variables such as age/sex, body mass, metabolic rate, genetics, health condition, frequency of administration, dosage, as well as co-administered drugs affect the length of time Cyclobenzaprine stays in the body. Discussed below are the roles these variables play.

Age / Sex

Aging naturally slows down metabolism. A younger individual may expect to get cleared of Cyclobenzaprine traces in usually 4 days. For elderly individuals, contrarily, it can take longer – usually twice as long – for the drug to process out of the body. With old age comes slower metabolism and overall poorer physiological function, so drugs are emitted much slower as they tend to linger longer in their system. It has also been found that the drug’s steady-state plasma concentrations were twice as high in elderly individuals as in younger individuals following the same oral doses of 5mg.

Body Mass

Weight also impacts the speed of eliminating the drug out of the system. Larger individuals have larger systems, and it helps metabolize Flexeril with more efficiency, so they are able to flush it out faster than smaller ones due to the latter’s smaller systems. The amount of body fat an individual has also plays a role here, and a higher body fat percentage helps speed up elimination.

Metabolic Rate

Metabolic rates are relevant in this case, too. Fast metabolizers eliminate Flexeril faster and more efficiently than slow metabolizers. While metabolic rates do not usually play a significant role in drug elimination, they still may have a small impact.



An individual’s race and genetic makeup can also impact drug metabolism. Certain allelomorphs are associated with enzymes (such as CYP3A4) that may alter Cyclobenzaprine metabolism and elimination. Depending on which allelomorph one has, elimination may see some help in facilitation or may be delayed. Its impact may not be of substantial clinical significance, though.

Health Condition

Individual health conditions, specifically health conditions that affect liver function, may also influence the amount of time Flexeril stays in the system. For instance, someone with an impaired hepatic function will process and eliminate the drug significantly slower than someone with a healthy liver as the liver helps with the elimination of the drug.

The Frequency/Duration of Drug Administration

Besides individual differences, how much Cyclobenzaprine is taken for treatment, and for how long, can influence its elimination speed. It is expected to stay longer in the system if a person takes it more frequently. For instance, it will stay longer in someone who takes it thrice a day than someone who takes it once a day. It has been found that when taken thrice a day, Cyclobenzaprine accumulates to nearly four times within plasma than a single-dose administration.


A man is searching information about Flexeril pills.

Drug Dosage

Taking a higher dosage of Cyclobenzaprine means it will stay longer in the body after cessation. The most commonly prescribed dosage is 5mg administered thrice a day, but higher dosages, like 10mg thrice a day, for example, will result in slower elimination of the drug due to increased potency and essentially double the amount of medication taken.

Co-administered Drugs

Taking other drugs, including alcohol, during a Cyclobenzaprine treatment may affect its excretion. Depending on how Flexeril interacts with the co-administered drugs (or alcohol, which can be dangerous when mixed with Flexeril), the speed may be decreased or increased. CYP3A4 inhibitors, for instance, may slow its elimination down, while CYP3A4 inducers may result in faster elimination. It’s always best to contact a doctor and heed their advice regarding taking other medications at the same time as interactions will be different on a case-to-case basis.

Flexeril Detox and How to Remove Flexeril From System Safely?

Eliminating Cyclobenzaprine from one’s system is usually not something worrisome. However, if in a situation that requires immediate removal of the drug from the system, one should consult and get in contact with a detox specialist and medical professional. They may carry out a Cyclobenzaprine drug test, and should be able to help one get the treatment or medical detox that would suit them best, considering the fact that several variables such as individual differences affect how long Flexeril stays in the system. Furthermore, one should always consider possible Flexeril side-effects.

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Published on: November 30th, 2018

Updated on: April 29th, 2024

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