Flexeril (cyclobenzaprine) generally starts working within about an hour after taking it, MedScape explains. It remains active in the bloodstream for about 12 to 24 hours.
It can remain in your system for much longer, up to several days.
Flexeril is a muscle relaxant medication prescribed to treat muscle spasms that occur as a result of painful and acute musculoskeletal conditions. The active ingredient, cyclobenzaprine, is chemically similar to tricyclic antidepressants (TCAs).
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While the mechanism of action is not entirely understood, it is known that Flexeril acts like a central nervous system (CNS) depressant. It slows down some CNS activity and nerve firings to calm muscle spasms and related pain.
Flexeril is an effective treatment for pain and muscle spasms for a relatively short period of time — usually between four and 12 hours. It can remain in the bloodstream for much longer — up to a few days.
Even if it is still present in the system, it may seem like it isn’t working. This can encourage people to take too much.
Flexeril is only designed to be taken for a short timeframe. The U.S. Food and Drug Administration (FDA) recommends that cyclobenzaprine is only to be prescribed for a maximum of two to three weeks.
When taking Flexeril, it is important to follow medical recommendations to minimize potential complications.
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Effectiveness vs. Elimination Timeline
As an immediate-release formulation, cyclobenzaprine is generally given in 5 mg doses three times per day. Doses should not exceed 30 mg per day, per PDR, the Prescriber’s Digital Reference.
The extended-release formulation of Flexeril may be dosed twice per day in 15 mg doses.
The immediate-release formulation generally remains effective for about four to six hours, while the extended-release can work for about 12 hours on pain and muscle spasms.
Flexeril has a long and variable half-life. It can stay in the body long after its effectiveness has worn off.
According to BioMed Research International, cyclobenzaprine has a half-life that may range from eight to 37 hours for the immediate-release version and between 32 and 33 hours for the extended-release version.
A half-life for medication is the length of time it takes for half of the drug to be broken down by the body. With this in mind, Flexeril can potentially stay in the body for up to three days.
It may seem as if Flexeril is no longer working, which may make it tempting to take more. If more is taken, it can trigger a wide range of side effects, not the least of which is the potential for overdose.
A cyclobenzaprine overdose can come on suddenly and rapidly. The U.S. National Library of Medicine (NLM) explains that the most common side effects are tachycardia (irregular heart rate) and drowsiness. Additional Flexeril overdose side effects can include the following:
- Nausea and vomiting
- Chest pain
- Muscle weakness
- Slurred speech
- Mental confusion
- High blood pressure
The U.S. Drug Enforcement Administration (DEA) published in 2010 that there were more than 12,000 emergency room visits that involved cyclobenzaprine. Over 10,000 case mentions of exposure to cyclobenzaprine were reported to the American Association of Poison Control Centers.
Misuse of Flexeril or combining it with other drugs or alcohol can amplify the potential side effects as well as the risk for fatal overdose.
Factors Impacting Metabolism
The length of time Flexeril stays in your body will depend on several factors. Not everyone metabolizes drugs the same way.
Cyclobenzaprine is excreted through the kidneys, so individuals with kidney damage or less function in the kidneys may struggle to process the drug out as quickly.
The FDA publishes that Flexeril takes longer to process out of the elderly and those with hepatic impairment; therefore, dosages should be lowered for these populations.
As you age, your metabolism slows down. This means that it can take longer for a drug to process out of the body.
Hepatic impairment involves the liver — which is the primary organ involved in flushing toxins out of the body — so issues with the liver can impede Flexeril metabolism. This slows the rate at which it is processed out of the system.
Other things that can influence how long Flexeril will stay in the system and how the drug is metabolized can include:
Crushing and snorting or chewing Flexeril tablets can interfere with the way the drug is meant to be metabolized. This can impact the speed at which it processes out of the system
Taking other drugs or drinking alcohol with Flexeril can exacerbate the effects of each substance. This also impacts drug metabolism and the rate at which Flexeril will process out of the body
The presence of a medical or mental health disorder can change how a drug is metabolized and how long it may stay in the system
Sex, age, and race can all impact drug metabolism
Tolerance and Drug Dependence
Another factor is based on how much and how often you take Flexeril.
When you take a drug that interacts with brain chemistry, your brain can get used to certain amounts of it. This means that you can become tolerant to the drug, and it will take more of it to feel the same effects. This can lead to escalating dosages of Flexeril, and more taken in between doses. Flexeril can then build up in the body, and it can take even longer for it to process out.
Physical drug dependence may form with regular use of Flexeril, which can lead to difficult withdrawal symptoms when it leaves the body. These symptoms may include malaise, nausea, and headache.
The more often you take Flexeril, the longer you’ve been taking it, and the higher the doses taken each time, the more significant the level of dependence will be. Higher levels of dependence mean it will take longer to leave your system.
Variations of Use
Flexeril is not intended for use longer than two to three weeks. It is generally prescribed as an adjunct treatment to be used in conjunction with other methods.
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Dependence is a precursor to substance abuse and addiction, for some people. Specialized treatment programs involving medical detox can help manage addiction while allowing Flexeril to process safely out of the system.
Other drugs including TCAs, MOAIs (monoamine oxidase inhibitors), SNRIs (serotonin and norepinephrine reuptake inhibitors), and SSRIs (selective serotonin reuptake inhibitors) can interact negatively with Flexeril. It is important that Flexeril is completely processed out of the body before introducing one of these medications.
The length of time Flexeril lasts in the system will vary from person to person. A medical professional can give you the best idea of a specific timeline for your unique situation.
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(April 2013). Flexeril. U.S. Food and Drug Administration. from https://www.accessdata.fda.gov/drugsatfda_docs/label/2013/017821s051lbl.pdf