Back injuries are very painful and cause debilitating muscle spasms. Doctors often prescribe muscle relaxants for these spasms to help relieve the pain. Cyclobenzaprine (Flexeril) is one such muscle relaxant. This drug can be very helpful for short term acute pain. Taken longer term or for recreational purposes, however, cyclobenzaprine can be detrimental or even life-threatening. Safe Harbor Treatment Center will explain what cyclobenzaprine is and how it is used, what effects it has on the body, and why it may be hazardous to health.
Flexeril is a muscle relaxant that is prescribed to reduce muscle spasms caused by acute injuries. The drug was first produced in 1961 with the intent to use it as an antidepressant since it is similar in structure to tricyclic antidepressants.1 While this use of Flexeril was abandoned when safer antidepressants were discovered, it was found to be effective as a muscle relaxant.
The United States Food and Drug Administration (FDA) approved Flexeril for use as a skeletal muscle relaxant in August of 1977, and it continues to be approved for this use today. There were 28.4 million prescriptions for Flexeril dispensed in the United States in 2017.2 Flexeril is taken orally as a tablet or capsule. It is approved for adults and adolescents age 15 or older. The extended-release versions of this drug are only approved for use by adults. Flexeril was designed for short term use, so it is not typically used to treat chronic pain.
This drug acts on the central nervous system, possibly at the brain stem level, to relieve muscle spasms. It does not work directly on the muscles and joints, but rather it calms the nerve signals from the brain that cause muscle spasms and pain. The drug helps to calm muscle function and improves mobility.3 While the mechanisms involved are unclear, it is theorized that Flexeril boosts norepinephrine levels while binding to the serotonin receptors in the brain to reduce pain.4
The first brand name assigned to cyclobenzaprine was Flexeril, and many people still know the drug by this name. This brand name for cyclobenzaprine has been discontinued in the United States, however, so physicians no longer prescribe Flexeril. The drug is available through its generic name, cyclobenzaprine, or through the following brand names:
Flexeril is classified as a skeletal muscle relaxant by the FDA. While this drug is very similar in structure to tricyclic antidepressants such as Amitriptyline, it differs slightly, making it not useful as an anti-depressant. Even though Flexeril has mechanisms that can cause physical drug dependence, the DEA does not consider it a drug with a high risk for abuse or addiction, and it is not listed on the controlled substance schedules.
Flexeril is used to relieve muscular pain and muscle spasm that result from acute, painful musculoskeletal conditions, such as back injuries or during recovery from surgeries. It is usually combined with an over the counter pain reliever, such as acetaminophen or ibuprofen, as well as rest and physical therapy. Some doctors use Flexeril to treat fibromyalgia. Using a lower dose of the drug as a sleep aid regularly appears to lessen pain and improve sleep in people who have fibromyalgia without increasing fatigue levels during the day.5 This use of Flexeril is considered “off-label,” meaning that the FDA has not approved Flexeril for the treatment of fibromyalgia. Other off-label uses of Flexeril include the treatment of:
Temporomandibular joint disorder (TMJ)
Tinnitus (ringing in the ears)
Even though the DEA has not classified this drug as a controlled substance, some people do misuse or abuse Flexeril . Indicators show that people who do abuse Flexeril do so by either increasing the dose or combining it with other drugs to produce a relaxing or sedative effect.6 The drug is taken either orally or snorted through the nasal passages to create this effect. Cyclobenzaprine is not typically sold on the street and does not have a significant street value. Abusers already have prescriptions for the drug or obtain it from relatives and friends who have prescriptions. It does have street names, however, including:
Based on data from the American Association of Poison Control Centers, Flexeril has had the highest number of case mentions and the highest number of reported single exposures out of all muscle relaxers in recent years. The chart below shows case mentions, and single exposures for Flexeril from the National Poison Data System Annual Reports for the years 2012 to 2018. This data is presented as the percent of incidents involving Flexeril as compared to the total number of incidents involving muscle relaxers.7
The chart shows that case mentions and single exposures to Flexeril have remained reasonably stable at around 40 percent of all cases involving muscle relaxers. Also, single exposures to Flexeril have held steady at around 40 percent of all case mentions. These numbers tell us that Flexeril abuse is quite high when compared to other muscle relaxers and that about 40 percent of all cases of cyclobenzaprine ingestion occur with the drug alone, with the other 60 percent being in combination with other medications. It is unclear how many of these incidents involve suicide attempts, but cyclobenzaprine has the potential to cause harm to the body great enough to require intervention for toxicity.
Because Flexeril is just a little bit different in structure from the tricyclic antidepressants, it is not considered to be an addictive drug. It is meant for short term use, however, and long-term use or abuse for chronic conditions can lead to some withdrawal symptoms. Stopping use of cyclobenzaprine abruptly can lead to:8
Some individuals who have been prescribed cyclobenzaprine for acute pain take the medication for longer periods than what it was intended. This can lead to the development of adverse side effects while having no increased benefit, as research shows.8 It is recommended that if an individual has to stop the use of Flexeril, they should decrease the dose gradually under the supervision of a physician, rather than just quitting “cold turkey.” This is not a drug that causes long term addiction, however, and treatment for addiction is not typically required. Those who abuse other drugs along with cyclobenzaprine may require treatment for their addictions to other drugs, such as alcohol.
Because Flexeril is a muscle relaxant, its side effects relate to this relaxant effect. Common side effects include:9
These side effects are less common but still mild:
Unpleasant taste in the mouth
Abdominal pain or discomfort
Serious side effects of Flexeril include:
Cyclobenzaprine relieves pain by acting on the serotonin receptors of the brain. Because of this, it can interact with other medications that also work on the serotonin receptors. If cyclobenzaprine is combined with other serotonin enhancing drugs, it can cause Serotonin Syndrome. This condition occurs when excess serotonin builds up in the body. It is a severe condition and can be fatal if not treated quickly.
Since Flexeril is not an anti-depressant but is chemically similar to tricyclic antidepressants, it may be prescribed together with anti-depressants that also increase serotonin levels on the body. If the dosages of both drugs are not carefully controlled, serotonin can build up in the body from the multiple medications, causing serotonin syndrome.
Rapid heart rate
High blood pressure
Loss of coordination
These symptoms of severe serotonin syndrome can be fatal:
Cyclobenzaprine is especially prone to interactions with monoamine oxidase (MAO) inhibitors. The risk of serotonin syndrome is higher if Flexeril is used with one of these medications, including:
Cyclobenzaprine also interacts with any drug that slows the brain’s processes, including alcohol, barbiturates, benzodiazepines, and opioids.11 Reactions with alcohol are common in incidences of Flexeril overdose.
Because Flexeril has a long half-life, from eight to 36 hours, it should not be used with elderly patients. This is because the drug is processed in the liver, and older adults have slower metabolism in the liver. Due to this, the drug stays in the system of older adults longer and tends to build up until average concentrations of the drug are 1.7 times higher than that of younger adults. This can lead to an increased risk of dangerous side effects in older adults.12
It is possible to overdose on Flexeril and, although rare, it can be fatal. When an overdose is intentional, cyclobenzaprine is often combined with alcohol or another depressant. Toxicity from a cyclobenzaprine overdose develops quickly, so anyone who is suspected of taking an overdose of cyclobenzaprine should receive immediate medical attention.
The symptoms of Flexeril overdose include:13
Tachycardia (rapid heartbeat)
Ataxia (poor coordination)
High blood pressure
More severe symptoms are potentially fatal. These include:
Low blood pressure
Cyclobenzaprine overdose can lead to Neuroleptic Malignant Syndrome. This condition results from a reaction to neuroleptic medication. Symptoms of neuroleptic malignant syndrome are:
Neuroleptic malignant syndrome must be diagnosed and treated quickly, or it can be fatal. It is vital to receive immediate emergency medical care if the symptoms above appear after an overdose of cyclobenzaprine.
Certain health conditions may be made worse by taking cyclobenzaprine, so those who need to use a muscle relaxant should check these precautions before taking it. Individuals who have the following health conditions may experience complications if they take this drug:14
Cyclobenzaprine may cause rapid or irregular heartbeats, so it may be dangerous for individuals who have an arrhythmia, heart blockages, congestive heart failure, or who have had a recent heart attack.
As hyperthyroidism causes rapid heartbeat, Flexeril may increase heart rate to dangerous levels if hyperthyroidism is present.
Cyclobenzaprine is processed in the liver, so individuals who do not have adequate liver function may not be able to process the drug, leading to a build-up in the body and possible toxic levels. Those who have mild liver disease may be able to take lower doses of Flexeril if necessary. Sufferers of moderate to severe liver disease should not take this drug.
Glaucoma causes increased pressure inside the eye. This is also a side effect of cyclobenzaprine, so taking the drug when glaucoma is present could increase intraocular pressure to dangerous levels.
Those who abuse or misuse drugs and have these health conditions may make their conditions worse by taking cyclobenzaprine. Even though this drug has a relaxing effect, it’s actual effect on the body is to increase heartbeat and fluid pressures, so it is not safe to take for an individual with other health conditions.
Flexeril is a muscle relaxant that can be helpful to people with acute pain from back injuries or other short-term muscular conditions. It has some significant side effects, however, and can be dangerous when abused or misused. Anyone who has taken too much Flexeril or taken it in combination with other depressant drugs should seek medical help immediately.