By Rosanna Sutherby, PharmD 

 Medically reviewed by Alex Yampolsky, PharmD

Robaxin (methocarbamol) and Amrix (cyclobenzaprine) are brand-name muscle relaxants used alongside rest and physical therapy to relieve muscle pain and stiffness due to muscle injuries, such as muscle strains (an injury to the ligaments and capsule of a joint) or sprains (an injury to muscles or tendons).

Both medications are taken orally (by mouth). However, Robaxin also comes as an intravenous injection (IV) that may be given into your muscle or vein.123

DailyMed. Label: Robaxin- methocarbamol injection.

Robaxin and Amrix treat muscle pain and stiffness related to muscle strains or sprains and are similar in some ways.45

However, while generally seen as similar by healthcare providers, Robaxin and Amrix carry some notable differences, such as differences in active ingredients, dosing requirements, side effects, and off-label uses.

This article will highlight the similarities and differences between Robaxin and Amrix.

Dusan Stankovic / Getty Images

Understanding Muscle Relaxants

Muscle strains or sprains can be caused by specific body movements, such as lifting heavy objects, jumping, throwing, or falling. Using muscles repeatedly may also cause strains or sprains.

Activities that may increase your risk for this type of injury include playing sports such as rowing or tennis or sitting or standing in poor posture for extended periods.6

Muscle relaxants categories include skeletal muscle relaxants and smooth muscle relaxants.

Robaxin and Amrix are both skeletal muscle relaxants.45 Other skeletal muscle relaxants are Soma (carisoprodol) and Skelaxin (metaxalone).7

As with other skeletal muscle relaxants, Robaxin and Amrix are used along with rest and physical therapy to relieve symptoms of muscle strains or sprains.

Healthcare providers often recommend treatments such as resting and applying ice to the injured muscle before prescribing muscle relaxants.6

They may also refer you to physical therapy. Generally, skeletal muscle relaxants, like Robaxin and Amrix, should be used for short courses of treatment.

If your healthcare provider prescribes a muscle relaxant for you, they’ll likely have you continue these other treatment methods along with your medication.

Skeletal Muscle Relaxants

Skeletal muscle relaxants help skeletal muscle pain and stiffness in muscles that help your body move.7

Examples of skeletal muscles include the following:

• Hamstring (muscles in the back of your thigh)

• Back muscles

• Calf muscles

Smooth Muscle Relaxants

Smooth muscle relaxants help relax muscles you can’t control.

For example, healthcare providers may prescribe Bentyl (dicyclomine) for muscles in the digestive tract for people with irritable bowel syndrome (IBS).89

What Is Robaxin?

Administered as an oral table or via IV injection, Robaxin is a prescription drug that's Food and Drug Administration (FDA)–approved to relieve pain and discomfort, such as stiffness, due to muscle injuries, including strains and sprains.23

Robaxin is used along with rest, physical therapy, and other measures. It is approved for use in people 16 years and older.23

The generic form of Robaxin is methocarbamol. Methocarbamol is available for administration as an oral tablet or IV injection.1011

Off-Label Uses

In addition to its FDA-approved uses, healthcare providers may prescribe Robaxin for off-label uses. Off-label use is when a drug is prescribed for a condition or at doses different from its FDA-approved uses.

Robaxin may be used off-label to relieve pain related to the following conditions:1213

• Short- or long-term back pain

• Inflammatory arthritis, such as rheumatoid arthritis (RA)

• Fibromyalgia (a chronic condition causing pain throughout the body)

• Myofascial pain (pain in the musculoskeletal system)

• Rib fractures

• Hip or knee replacement surgeries

How Does Robaxin Work?

Robaxin does not work directly on muscles. Instead, it works by slowing down activity in your central nervous system (CNS), which is made up of your brain and spinal cord.414

It is thought that your body may produce muscle spasms after an injury to prevent specific muscles from moving, avoiding further damage to the area.15

These spasms can result in muscle pain and stiffness. By slowing down activity in your CNS, Robaxin reduces signals from your brain that promote muscle spasms.

As a result, this causes more relaxed muscles and pain relief.2

Robaxin starts working about 30 minutes after you take a dose. It may take up to two hours to feel the full effects of the medication.

These effects typically last about four to six hours.4

What Is Amrix?

Amrix, containing the active ingredient cyclobenzaprine, is used with rest and physical therapy to relieve pain and stiffness related to muscle spasms due to muscle injuries in adults, including strains and sprains.1

Amrix is an extended-release (ER) oral capsule,1 while generic cyclobenzaprine comes in both ER capsule and immediate-release (IR) oral tablet forms.1617

Off-Label Uses

Healthcare providers may prescribe cyclobenzaprine off-label to treat the following:181920

• Trouble sleeping due to fibromyalgia

• Myofascial pain

• Certain symptoms of post-traumatic stress disorder (PTSD)

How Does Amrix Work?

Like Robaxin, Amrix works by slowing down activity within the CNS, which reduces signals from your brain that stimulate muscle spasms.5

Amrix also helps relieve muscle spasms by blocking specific serotonin receptors, which results in muscle relaxation and pain relief.

Amrix starts working about seven to eight hours after taking a dose, and it may take up to four days to feel the full effect of the drug. These effects typically last a full day.1

Dosing Guidelines

The following are dosing guidelines recommended by the manufacturers of Robaxin and Amrix.

Your healthcare provider will prescribe a dosage specific to your condition. Be sure to take Robaxin or Amrix precisely as your healthcare provider recommends:123

*This form is only available as the generic methocarbamol.

Guidance for Safe Use

The following outlines essential guidance regarding the use of Robaxin vs. Amrix.

Overall, know that Robaxin injection and Amrix oral capsules are prescribed for short-term use.13

Robaxin

The maximum recommended length of treatment for Robaxin injection is three days.3

After this, your healthcare provider may prescribe the oral form of the medication (methocarbamol tablets), which may be taken long-term.

Tablets are typically taken three to six times per day, depending on the strength your healthcare provider prescribes.2

Be sure to take Robaxin precisely as your healthcare provider recommends. If you miss a dose, do not take two doses to make up for the missed dose.

Take your missed dose as soon as you remember.

However, if it’s almost time for your next dose, skip the missed dose and continue following your regular dosage schedule.4

Amrix

With Amrix, the recommended length of treatment is two to three weeks. Your healthcare provider will tailor your dosage based on the severity of your condition.

Amrix is an ER capsule, which means the medication is released into your body slowly over time. Because of this, Amrix oral capsules only need to be taken once per day.

Amrix capsules should be swallowed whole without chewing or crushing them. If you have trouble swallowing capsules, open Amrix capsules and mix the contents in 1 tablespoonful of applesauce.

Then, consume all the applesauce right away without chewing. You may drink water afterward to ensure you swallowed all the applesauce with the Amrix.1

Your healthcare provider will likely recommend that you take Amrix at the same time each day. If you miss a dose, take it as soon as you remember on the same day.

Refrain from doubling up Amrix capsules to make up for a missed dose.

Following your healthcare provider’s instructions for taking Robaxin or Amrix is essential. Do not stop taking either medication or change your dose without checking with your healthcare provider.5

Comparing Efficacy

Regarding which is a more effective drug, both Robaxin and Amrix were approved for use a long time ago, so there aren’t many recent studies directly comparing the two.

A review of studies compared several skeletal muscle relaxants in treating muscle spasms, including methocarbamol and cyclobenzaprine (the active ingredients in Robaxin and Amrix).

The review found that methocarbamol and cyclobenzaprine worked better than placebo (no medication) to relieve pain and tenderness and improve motion.

When comparing side effects, people who took cyclobenzaprine were more likely to report drowsiness than those who took methocarbamol.21

Alternative Treatment Options

If you have a muscle injury, such as a muscle strain, you should know alternative ways to treat your condition besides muscle relaxants.

Mild muscle strains can often heal independently and don’t require prescription medications.

Alternative ways to manage muscle strains include:22

• Resting the injured muscle: You may need to rest your muscles for the first few days after your injury. This may mean refraining from playing sports that may have caused the injury. Unless your healthcare professional recommends it, it is optional to immobilize the muscle completely. This is because doing so may lead to or worsen stiffness.

• Applying ice to the injured area: Applying ice soon after a muscle injury can help reduce swelling and relieve pain. You can apply ice as frequently as you would like but for no longer than 15 minutes at a time.

• Physical therapy: If needed, your healthcare provider may refer you to a physical therapist as part of your treatment for your muscle injury. Physical therapy can provide gentle stretching and strengthening of the muscles. This can help with healing and help prevent future injuries.

Side Effects & Precautions

Like all medications, Robaxin and Amrix may cause side effects in some people. Therefore, be sure to monitor yourself or a loved one closely for the emergence of side effects.

It’s important to let your healthcare provider know if you have bothersome or severe side effects from Robaxin or Amrix. They can help you find ways to manage your side effects.

They may also recommend a different treatment option if your side effects are severe or life-threatening.

Common Side Effects of Robaxin

Some common side effects of Robaxin include:23

• Sleepiness

• Dizziness

• Headache

• Nausea

Common Side Effects of Amrix

Common side effects of Amrix include:1

• Sleepiness

• Dizziness

• Dry mouth

• Constipation

• Nausea

Severe Side Effects of Robaxin

Severe side effects of Robaxin can include:23

• Seizure

• Low white blood cell count

• Jaundice (yellowing of the skin and eyes caused by diseases of the liver or biliary tract)

Severe Side Effects of Amrix

Severe side effects of Amrix include:1

• Serotonin syndrome (SS, a buildup of serotonin in your system)

• Tachycardia (rapid heart rate)

• Cardiac arrhythmia (irregular heart rhythm), which may lead to heart attack or stroke

Precautions

People allergic to Robaxin or any of its ingredients should not take Robaxin tablets or use Robaxin injections.23

Additionally, individuals who have kidney conditions should not get the Robaxin injection.3

Amrix capsules should be avoided in the following cases:1

• While taking a monoamine oxidase inhibitor (MAOI, a class of antidepressants) or within 14 days of stopping MAOI treatment

• If a person has had a recent heart attack or has other heart conditions, such as irregular heart rhythm or congestive heart failure (CHF)

• If a person has hyperthyroidism (an overactive thyroid gland)

• If a person is allergic to Amrix or any of its ingredients

Summary

Muscle strains or sprains happen when a muscle or ligament is pulled or stretched too far.

Common causes of muscle strains or sprains include participating in certain sports, lifting heavy objects, and poor posture.6

Muscles commonly affected by strains or sprains include the muscles of your:

• Hamstring       

• Back

• Calf

Robaxin and Amrix are skeletal muscle relaxants approved to relieve pain and stiffness caused by muscle injuries, such as muscle strains or sprains.

Although the two drugs are similar in many ways, they differ. For example, Amrix is approved for adults only, while Robaxin may be used in adults and children 16 years or older.123

Healthcare providers may prescribe Amrix for up to three weeks. Robaxin may be used long-term if you need additional treatment.45

Both drugs are similar in efficacy. However, if you prefer to take fewer daily doses, Amrix might be a better option.

Robaxin and Amrix have similar common side effects, but Amrix may cause more drowsiness than Robaxin. Also, if you have certain health conditions, such as heart or thyroid issues, Robaxin may be a safer choice for you.123

Talk with your healthcare provider to determine which drugs are right for you.

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By Rosanna Sutherby, PharmD
Rosanna Sutherby, PharmD, is a freelance medical writer and community pharmacist with over 20 years of experience in medication review, counseling, and immunization.