By Edmund Murphy

Last updated: 04 April 2024 & medically reviewed by Dr. Kimberly Langdon

Cyclobenzaprine (Flexeril/Amrix) is a prescribed medication used to treat mild to moderate muscle pain and musculoskeletal disorders. As with all drugs, the length of time it is detectable in the system varies depending on multiple factors.

Key takeaways:

  • Cyclobenzaprine, sold under the brand name Flexeril and Amrix, is hard to detect in most standard drug toxicology tests.

  • Hair follicle testing is often able to show precisely when drugs were taken up to 90 days, however, in the case of Flexeril, hair tests can only pinpoint use up to three days after last use. 

  • As a rough estimate, it usually takes 5.5 days x the half-life of the drug for it to be fully out of your system.

Is Flexeril detectable in your system?

Cyclobenzaprine, sold under the brand name Flexeril and Amrix, is hard to detect in most standard drug toxicology tests. This is due to most drug tests screening for specific drug metabolites which aren’t present in Flexeril.

However, those who abuse cyclobenzaprine often do so in conjunction with other central nervous system depressants such as opioids, alcohol, and benzodiazepine. These substances are often picked up in urine, blood, saliva, and hair drug tests.

Flexeril detection time by drug test

As already mentioned, Flexeril and Amrix won't show up on most drug screening tests unless specifically tested for. Testing for cyclobenzaprine based drugs is rare as they are considered nonaddictive by the DEA. However, Flexeril can be detected in blood, hair, saliva, and urine if targeted in a toxicology test.

Urine tests

Flexeril shows up most consistently and for the longest in urine testing. Flexeril can be detectable in urine between 5 and 13 days after last use.

Blood tests

Blood testing for Flexeril is extremely rare as the substance is only detectable up to 4 hours after last use.

Saliva tests

Flexeril and other cyclobenzaprine based drugs are not detectable in saliva.

Hair tests

Hair follicle testing is often able to show precisely when drugs were taken up to 90 days after last use. however, in the case of Flexeril, hair tests can only pinpoint use up to three days after last use. 

When will Flexeril be out of your system?

The length of time a drug is in your system depends on its half-life, that is how long it takes for half of the drug to be processed and absorbed into the body. 

Flexeril has a relatively long half-life of between 1 and 3 days.[1] As a rough estimate, it usually takes 5.5 days x the half-life of the drug for it to be fully out of your system. This puts Flexeril at between 5.5 days and 16.5 days for it to be eliminated from the system.[1]

Read here to see how long other substances stay in your system. 

Factors that affect how long Flexeril stays in your system

There are many factors that can influence the detection rate in a toxicology test and these can vary from person to person. 

Factors that influence Flexeril detection rates include:

  • Length of time the drug was used for
  • Quantity of the drug used
  • Purity of the drug (not applicable to cyclobenzaprine)
  • Whether other substances were taken (such as alcohol or benzos)
  • Body mass index (BMI)
  • Gender
  • Age (elderly people take longer to metabolize substances)
  • Metabolism
  • Pre-existing medical conditions (especially those involving the liver function or kidney function)