Muscle relaxers are specific prescription medications used to reduce or relieve muscle spasms, back pain, and general spasticity. If you or someone you know is taking muscle relaxers, it is crucial to understand how long they can remain active and detectable in the body for safe use. This is particularly true when it comes to avoiding potential interactions with other substances that may act as central nervous system depressants, like opioids or benzodiazepines.
Whether you use muscle relaxers from a prescription, or someone you care about is misusing cyclobenzaprine (Flexeril), methocarbamol (Robaxin), carisoprodol (Soma), baclofen, or any other powerful antispasmodic, knowing the half-life and metabolic behavior, along with average detection windows can help ensure safer use in all situations.
What Are Muscle Relaxers and Why Are They Prescribed?
Skeletal muscle relaxants are a type of prescription medication specifically designed to reduce the frequency and intensity of muscle spasms, spasticity, and associated musculoskeletal pain. Some of the most common agents include cyclobenzaprine, methocarbamol, carisoprodol, baclofen, and tizanidine, all of which have one or more retail brand names.[1]
Healthcare providers choose a muscle relaxant based on the clinical presentation of their patient, considering each drug’s side effect profile. This means they’ll consider the patient’s acute injury, chronic spasticity, or muscle pain, in comparison to the potential for side effects unique to each drug.
Many healthcare providers recommend short-term use only for muscle relaxers, typically two to three weeks, alongside other therapy methods and NSAIDs to help mitigate muscle pain without overexposure to the risk of dependence, which can be a serious concern when opioids or benzodiazepines are co-prescribed.
How Muscle Relaxers Are Metabolized
Most muscle relaxants are metabolized by the liver, which creates active or inactive metabolites that are then excreted by the kidneys as waste. While the general process is largely the same at a high level for all muscle relaxers, several factors can influence how quickly or slowly each drug is processed and removed from the body.
- Hepatic Function: Cyclobenzaprine and carisoprodol rely heavily on being metabolized by the liver.[2][3] Liver function that has been impaired by things like cirrhosis, hepatitis, or chronic alcohol use will prolong the half-life of most drugs that pass through it. This means the average half-life of cyclobenzaprine, which is 18 hours, can become drastically longer. This raises the risk of toxicity and dangerous sedation.
- Renal Function: Baclofen has a half-life of 2 to 6 hours and is primarily excreted unchanged in the urine, with minimal metabolism. [4]As a result, diminished kidney function or renal insufficiency due to chronic disease can cause the potential for drowsiness, hypotension, and respiratory depression to spike.
- Age & Body Composition: As individuals age, they generally exhibit a slower metabolism and decreased muscle mass, which can alter the distribution of volume. Even though a drug like methocarbamol has a half-life of 1 to 2 hours, a geriatric patient who takes it can experience extended sedation, increasing the risks of falls, compared to someone significantly younger.[5] Lower body weight tends to lead to higher plasma concentrations as well.
- Genetic Variations: Genetic polymorphisms in CYP2C19 or CYP3A4 can influence the baseline activity of drugs such as cyclobenzaprine and carisoprodol.[6][7] Poor metabolizers will experience extended half-lives, while fast metabolizers will feel the drug clear their system much more quickly, which can lead to diminished efficacy.
- Drug Interactions: Using opioids or benzodiazepines increases depression of the central nervous system, with SSRIs, SNRIs, and other drugs altering liver enzyme levels, which can alter the clearance of countless drugs.
Half-Life and Duration of Common Muscle Relaxers
Methocarbamol
Also marketed as Robaxin, methocarbamol has one of the shortest half-lives of just 1 to 2 hours.[8] Clinical effects can be measured to last 4 to 6 hours after a 1500mg “loading dose”, with a spike in muscle relaxation and sedation occurring within about 45 minutes.
Rapid metabolism in the liver produces inactive metabolites that are then filtered through the kidneys and excreted into the urine. Due to the rapid clearance, it’s not uncommon for patients to need 750mg to 1000mg every 4 to 6 hours to mitigate muscle spasms.[9]
Methocarbamol is detectable in urine for just 24 to 48 hours, in saliva for 1 to 2 days, and in hair for three months or longer. It can be difficult to detect in blood tests due to the rapid half-life.[10]
Carisoprodol
Carisoprodol is sold under the brand name of Soma, and has a half-life of just 2 hours, but is then metabolized into meprobamate, which has a half-life of 10 to 20 hours. As a result, the sedation and detection are extended.[11]
The effects typically peak within an hour and last for approximately 6 hours, based on a typical dose of between 250 mg and 350 mg, administered three times daily.[12] However, prescriptions usually don’t exceed 1200mg per day due to the abuse potential and possibility of meprobamate toxicity.
Both carisoprodol and meprobamate can be seen in the urine for 48-72 hours post-dose, with high doses visible for 6 to 7 days.[13] Blood and saliva detection is limited to 24 hours, whereas hair tests can be used for months.
Baclofen
Lioresal (Baclofen) has a half-life of 3-4 hours, requiring a dose to be taken every 6-8 hours to avoid peaks and troughs. Due to its rapid removal through the kidneys, steady-state is only achieved after 24-48 hours of consistent dosing.[14]
The usual dosages range from 5mg three times a day to a maximum of 80mg per day for extreme cases. Withdrawal symptoms will typically include rebound spasticity, pruritus, hypotension, and paresthesias, and can start as quickly as 24 hours after sudden discontinuation.[15]
Like most other muscle relaxers, Baclofen can be detected in saliva for approximately a day and in hair for several months. It can be seen in urine for 24 hours with average renal function.[16]
Treatment Options for Misuse and Safety
Overall, healthcare providers should assess the need for muscle relaxants in comparison to non-prescription options. Physical therapy, stretching, and the application of alternating heat and ice are effective treatments for reducing muscle pain and spasms, eliminating the need for medication. Over-the-counter NSAIDs like ibuprofen, acetaminophen, and aspirin can also offer pain relief and anti-inflammatory effects that can be of considerable help.
For those who are showing signs of misuse or potential dependence, getting a referral to a local addiction program or medical detox program, like The Freedom Center, can be the early intervention that sparks a successful recovery. With access to leading evidence-based modalities like cognitive-behavioral therapy and motivational interviewing, individuals are empowered to address underlying mental health conditions that may be fostering substance use disorder.
