How Long Does Methocarbamol Stay In Your System And Its Half-Life

Last Updated: September 30, 2021

Methocarbamol (Robaxin) is a centrally-acting skeletal muscle relaxant (SMR) and antispasmodic medication that has been around on the pharmaceutical market since the 1950s. Although the specific FDA indications for use are quite vague and almost certainly outdated, it is mainly approved and indicated as an adjunct in the treatment of acute musculoskeletal pain and muscle spasms secondary to local injury or muscle sprain. Additionally, numerous off-label uses have been investigated for the treatment of a range of painful musculoskeletal conditions. This article will focus on pharmacokinetics and pharmacodynamics of this medication. In particular information about Methocarbamol MOA, Methocarbamol half life, how long it stays in the user’s system, and the variables which can affect these factors. As well as Methocarbamol Drug Tests and ways in which it can be safely removed from a user’s system.

Methocarbamol Mechanism Of Action

As mentioned above, Methocarbamol is an antispasmodic agent that can be used for the treatment and relief of pain and acute muscle spasm caused by local tissue trauma or muscle strains. Unfortunately, antispasmodic drugs which also include medications such as Carisoprodol, Chlorphenesin, Chlorzoxazone, Cyclobenzaprine, Metaxalone, and Orphenadrine are less well-studied and information about their exact mechanism of action in relief and treatment of muscle spasms and associated musculoskeletal pain remains unknown. Several theories, however, exist about Methocarbamol MOA. According to one proposed mechanism, antispasmodic drugs act at the level of the brain stem, more specifically the midbrain reticular bundle. Here, antispasmodics cause inhibition of the neurons which are involved in the generation of the spasmic contracture of muscles that cause pain and also decrease muscle tone. Additionally, what is known is that this medication has no direct effects at the motor nerve fiber, motor nerve end plate, or skeletal muscle contractility, meaning that it does not directly relax skeletal muscle and will not depress neural conduction, neuromuscular transmission, or muscle excitability.

As such, this medication will produce skeletal muscle relaxation by blocking spinal polysynaptic reflexes, without loss of consciousness or significant sedation.

How Long Does It Take for Methocarbamol To Work

After oral administration of either 500 or 750 milligram tablets of this medication, it is completely absorbed through the gastrointestinal tract and is widely distributed in body tissues with high concentrations occurring in the kidney, liver, and the central nervous system. Peak plasma concentration is reached within 2 hours, however, the onset of its antispasmodic and pain relieving effects can be noted within 30 minutes of oral administration. Patients can experience these effects for 4-6 hours.

A man holds blue pills in his hand.

Methocarbamol Half-Life And Metabolism

The “half-life” or “elimination half-life” of a drug refers to the length of time required for the amount of a drug’s active substance in a patients body to be reduced to half of its initial dose in the body. The half life of a drug is generally constant under normal conditions, meaning that no drug-drug interactions have occurred which could alter the metabolism of a given drug and the individual taking the drug is of sound health. Under these conditions, within 1 half life of a drug, 50% of its initial concentration will be eliminated from a patients body. And within 4 to 5 half lives 94 to 97% of a drug will be eliminated, which means that the plasma concentrations of a given drug will be below a clinically relevant concentration and will be considered completely eliminated.

Hence, with regard to Methocarbamol half life, it is varied and ranges from 1 to 2 hours. It undergoes extensive hepatic metabolism by dealkylation, hydroxylation, and glucuronidation to inactive metabolites and is primarily excreted in the urine. It is also worth mentioning that 10-15% of this medication is excreted as an unchanged drug in urine.

How Long Does Methocarbamol Stay In Your System

As mentioned above the factor that governs how long does methocarbamol stay in your system is its half-life. As stated, Methocarbamol half-life is 1 to 2 hours. However, the medicine will be eliminated from an individual’s blood and saliva in approximately 5-10 hours with only trace amounts remaining. Inactive metabolites will be present in urine for a similar amount of time, ~5-10 hours. Lastly, although very rarely utilized as a method of detecting this drug, it can remain detectable in an individual’s hair follicle for up to 90 days.

Biologic Samples Duration of Detectability 
Urine 5-10 hours
Blood 5-10 hours
Saliva 5-10 hours
Hair Up to 90 days

It is always important to keep in mind that individual variation in age, gender, weight, genetics, concurrent illnesses, and duration of use can all affect these durations, and as such these time-frames are subject to individual variability.

Methocarbamol Drug Test

It is important to remember that Methocarbamol is not included in the regular list of substances that are commonly tested using commercial drug panels. However, more specific Methocarbamol drug tests done in research laboratories and forensic labs such as liquid and gas chromatography are able to detect this medication and its inactive metabolites in all biologic samples. As such, if traces of this drug will be detected in an individual’s biological samples, a valid prescription may be asked for.

There are several cases where prescription drugs cause a false positive when a person is tested for an illicit substance. One study describes that there are several categories of medication that come to mind. Thus, when considering whether Robaxinl may cause a positive on a drug test, it is essential to take this factor into consideration. While the study does not give a definite answer to whether or not a Robaxin false positive drug test may occur, it does state that analgesics have been shown to have such an effect. There may also be certain cases where Methocarbamol drug interactions may interfere with this particular factor as well.

Variables That Affect How Long Methocarbamol Stays In One’s System

The primary factor which governs how long the drug will stay active in an individual’s system is its elimination half-life, or once again the length of time required for the amount of a drug’s active substance in a patient’s body to be reduced to half of its initial dose. Numerous factors can have effects on the elimination half life of a medication. Below are several such factors that can have an effect on how long this medication stays in one’s system.

  • Physiologic Age: Older age is associated with slower rate of metabolism and overall decrease in health and organ function. This can lead to a decreased rate of elimination of drugs from one’s system.
  • Genetics: Individual variations in genetics and genetic polymorphisms can influence the rate of hepatic metabolism. The alteration in the rate of hepatic metabolism occurs through influence on the expression of specific cytochrome P450 enzymes that are involved in the metabolism of various drugs.
  • Volume of Distribution: Certain drugs have a propensity to make their way from central circulation and become deposited into peripheral tissues. Robaxin has a high volume of distribution into peripheral organs such as the liver and the kidney. Peripherally distributed drugs and “stored” rather than eliminated. This can prolong the duration of time a drug can stay in an individual’s system.
  • Metabolic Rate: Metabolism refers to the sum of all chemical processes that occur within a living organism in order to maintain life. Hepatic biotransformation of this medication is also such a chemical process. Thus, all factors that can influence the rate of metabolism whether they be related to genetics, sex, or pathologic processes can ifluence the duration of how long a medication will stay in one’s system.
  • Organ Function: The health of an individual’s heart, kidney, and liver are all essential to metabolism and elimination of a drug. Those individuals who suffer from cardiac, renal, and hepatic disease often have impaired abilities to metabolise and eliminate medications.
  • Dose and Frequency of Use: The amount of this medication that is taken for treatment, and duration of it use, can both influence the rate of its elimination. Chronic and frequent use of drugs can increase the amount of time these drugs stay in an user’s system.
  • Drug-Drug Interactions: The consumption of multiple drugs together has the propensity to cause various drug to drug interactions. Several types of drug to drug interactions can either speed up or decrease the rate of hepatic metabolism carried out by various cytochrome P450 enzymes. Through this mechanism the duration of how long a medication stays in one’s system can either be increased or decreased.

Methocarbamol Detox and How to Remove It From System Safely?

Detoxing from Robaxin is not particularly life-threatening, it is a matter of waiting several hours for our bodies to eliminate it on its own. In case of overdose in the setting of drug addiction and in the case of chronic misuse, or adverse reactions individuals might want to contact a medical professional and consider the possible alternatives or professional detox services. This helps to eliminate the chemicals from their body faster. People need to understand Robaxin contraindications and be cautious when using the drug. Individuals with Robaxin abuse problems should seek information about professional help in certified drug rehabilitation centers.

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Published on: January 29th, 2020

Updated on: September 30th, 2021