Side Effects and Risks of Muscle Relaxers

The potential side effects of muscle relaxers are related to their action of slowing activity within the nervous system (central nervous system depression).¹Baclofen. Package Insert. Advagen Pharma Limited; 2023 ^-10Methocarbamol. Package Insert. Granules Pharmaceuticals Inc.; 2023

Common Side Effects of Muscle Relaxers

The side effects are usually well-tolerated when used short-term at recommended doses, and include¹¹Cashin AG, Folly T, Bagg MK, et al. Efficacy, acceptability, and safety of muscle relaxants for adults with non-specific low back pain: systematic review and meta-analysis. BMJ. 2021;374:n1446. Published 2021 Jul 7. doi:10.1136/bmj.n1446: 

• Drowsiness
• Dizziness
• Fatigue
• Headache
• Dry mouth
• Constipation
• Nausea

Most people will experience mild to moderate sedation, which can impact daily activities like driving.

Other potential side effects of muscle relaxers

The following table lists the additional side effects associated with certain muscle relaxers.

Muscle relaxers impact each person differently, and while some people may experience more side effects, others may have fewer or no side effects.¹Baclofen. Package Insert. Advagen Pharma Limited; 2023^-10Methocarbamol. Package Insert. Granules Pharmaceuticals Inc.; 2023

Potential Risks and Serious Adverse Events Associated with Muscle Relaxers

Serious adverse events are rare, but the presence of certain underlying conditions or the use of concomitant medications increases the risk of these events.¹Baclofen. Package Insert. Advagen Pharma Limited; 2023^-10Methocarbamol. Package Insert. Granules Pharmaceuticals Inc.; 2023

Central nervous system depression 

Drowsiness and sedation can be severe enough to impair the mental and physical abilities of some people, especially older adults or people who take other sedating medications.¹Baclofen. Package Insert. Advagen Pharma Limited; 2023^-10Methocarbamol. Package Insert. Granules Pharmaceuticals Inc.; 2023

Using muscle relaxers with alcohol or cannabis increases the risk of sedation and impairment.¹Baclofen. Package Insert. Advagen Pharma Limited; 2023^-10Methocarbamol. Package Insert. Granules Pharmaceuticals Inc.; 2023

Liver toxicity 

Some muscle relaxers cause damage to the liver, and the extent of this damage varies depending on the medication. Liver injury can range from an increase in the levels of liver enzymes to severe liver toxicity and is associated with the following medications²Tizanidine. Package Insert. Advagen Pharma Limited; 2020^,5Chlorzoxazone. Package Insert. Aurobindo Pharma Limited; 2022^,9Dantrolene. Package Insert. Amneal Pharmaceuticals of New York LLC; 2022:

• Chlorzoxazone
• Dantrolene 
• Tizanidine 

In most cases, liver function returns to normal when the drug is stopped.²Tizanidine. Package Insert. Advagen Pharma Limited; 2020^,5Chlorzoxazone. Package Insert. Aurobindo Pharma Limited; 2022^,9Dantrolene. Package Insert. Amneal Pharmaceuticals of New York LLC; 2022

Withdrawal effects 

Withdrawal effects are possible when abruptly stopping certain muscle relaxers, and these effects are associated with long-term use and/or high doses of the following medications¹Baclofen. Package Insert. Advagen Pharma Limited; 2023^,7Soma. Package Insert. Meda Pharmaceuticals Inc; 2023^,8Diazepam. Package Insert. Mayne Pharma Commercial LLC; 2023: 

• Baclofen (intrathecal injection)
• Diazepam
• Carisoprodol

To prevent withdrawal symptoms, these medications are gradually tapered under the supervision of a doctor.¹Baclofen. Package Insert. Advagen Pharma Limited; 2023^,7Soma. Package Insert. Meda Pharmaceuticals Inc; 2023^,8Diazepam. Package Insert. Mayne Pharma Commercial LLC; 2023

When Muscle Relaxers Should Be Avoided in People with Underlying Conditions

The following table lists the medical conditions in which certain muscle relaxers have a higher risk of adverse events.

Close monitoring for adverse events is advisable if muscle relaxers are used in the conditions listed above.²Tizanidine. Package Insert. Advagen Pharma Limited; 2020^-4Cyclobenzaprine. Package Insert. Rising Health, LLC; 2020^,6Metaxalone. Package Insert. Amneal Pharmaceuticals LLC; 2022^,8Diazepam. Package Insert. Mayne Pharma Commercial LLC; 2023^-10Methocarbamol. Package Insert. Granules Pharmaceuticals Inc.; 2023 Doctors will gather a detailed medical history before prescribing a muscle relaxer.

Risks of Muscle Relaxers in Older Adults

The use of muscle relaxers in older adults carries a higher risk for side effects, such as sedation, confusion, urinary retention, or memory problems due to¹²2023 American Geriatrics Society Beers Criteria Update Expert Panel. American Geriatrics Society 2023 updated AGS Beers Criteria for potentially inappropriate medication use in older adults. J Am Geriatr Soc. 2023;71(7):2052-2081. doi:10.1111/jgs.18372:

• The body’s ability to process and clear medications declines with age, and older adults are more likely to have impaired liver or kidney function, which can enhance the sedating side effects of muscle relaxers. 
• Older adults usually take multiple medications and supplements, which may increase the risk of potentially dangerous drug interactions that increase the intensity of side effects of muscle relaxers.

It is advisable to discuss these risks with a doctor and consider non-drug therapies, such as massage therapy or strength training, if possible, before prescribing a muscle relaxer for this age group.¹²2023 American Geriatrics Society Beers Criteria Update Expert Panel. American Geriatrics Society 2023 updated AGS Beers Criteria for potentially inappropriate medication use in older adults. J Am Geriatr Soc. 2023;71(7):2052-2081. doi:10.1111/jgs.18372

Risks of Muscle Relaxers in Pregnant or Lactating Women

Muscle relaxers have not been well-researched in pregnant or lactating women, and the effects of these medications on the developing fetus or infant are largely unknown.¹Baclofen. Package Insert. Advagen Pharma Limited; 2023^-10Methocarbamol. Package Insert. Granules Pharmaceuticals Inc.; 2023

These medications should be used with caution and under the supervision of a doctor in cases of pregnancy or in women who are breastfeeding. Generally, alternative therapies are preferred due to the unknown safety risks of muscle relaxers in these individuals.¹Baclofen. Package Insert. Advagen Pharma Limited; 2023^-10Methocarbamol. Package Insert. Granules Pharmaceuticals Inc.; 2023

Addiction and Abuse Potential of Muscle Relaxers

Some muscle relaxers have the potential to be abused because of their calming, sedating, or even euphoric effects. Using the medication for purposes other than the intended use or taking it in excessive amounts can lead to both physical and psychological addiction or dependence.¹³Horsfall JT, Sprague JE. The Pharmacology and Toxicology of the 'Holy Trinity'. Basic Clin Pharmacol Toxicol. 2017;120(2):115-119. doi:10.1111/bcpt.12655 

Even when used at regular dose, the body can develop tolerance to certain muscle relaxers and eventually need a higher dose to have the original effect.⁷Soma. Package Insert. Meda Pharmaceuticals Inc; 2023^,8Diazepam. Package Insert. Mayne Pharma Commercial LLC; 2023

Muscle relaxers that are commonly associated with the risk of abuse and/or dependence include⁷Soma. Package Insert. Meda Pharmaceuticals Inc; 2023^,8Diazepam. Package Insert. Mayne Pharma Commercial LLC; 2023:

• Diazepam (Valium)
• Carisoprodol (Soma)

Tolerance to these medications is more likely to develop when used for periods longer than 2-3 weeks.⁷Soma. Package Insert. Meda Pharmaceuticals Inc; 2023^,8Diazepam. Package Insert. Mayne Pharma Commercial LLC; 2023

Risks of Combining Muscle Relaxers with Opioids

Due to the significantly increased risk of central nervous system depression, it is strongly advised to avoid combining muscle relaxers with opioid medication such as oxycodone or morphine. Research suggests that there is no improvement in pain relief when combining these medications, and the risks do not outweigh the benefits in most cases.¹³Horsfall JT, Sprague JE. The Pharmacology and Toxicology of the 'Holy Trinity'. Basic Clin Pharmacol Toxicol. 2017;120(2):115-119. doi:10.1111/bcpt.12655

Additionally, benzodiazepines, carisoprodol, and opioids (commonly referred to as the “Holy Trinity”) are avoided within the same prescription due to concerns about safety and the potential for abuse.¹³Horsfall JT, Sprague JE. The Pharmacology and Toxicology of the 'Holy Trinity'. Basic Clin Pharmacol Toxicol. 2017;120(2):115-119. doi:10.1111/bcpt.12655

Long-Term Use of Muscle Relaxers 

Muscle relaxers are not recommended for long-term use (more than 3 months). Research suggests that there is little to no improvement when used for long periods, and the risk of side effects or serious adverse events typically outweighs the potential benefit in the long term.¹¹Cashin AG, Folly T, Bagg MK, et al. Efficacy, acceptability, and safety of muscle relaxants for adults with non-specific low back pain: systematic review and meta-analysis. BMJ. 2021;374:n1446. Published 2021 Jul 7. doi:10.1136/bmj.n1446

See 6 Things to Know About Muscle Relaxants

When to Report Side Effects to A Doctor

When taking muscle relaxers, it is advisable to report any of the following side effects to a doctor¹Baclofen. Package Insert. Advagen Pharma Limited; 2023^-10Methocarbamol. Package Insert. Granules Pharmaceuticals Inc.; 2023:

• Blurred vision
• Fainting
• Confusion
• Urinary retention
• Allergic reactions

These side effects could be signs of a more serious reaction.¹Baclofen. Package Insert. Advagen Pharma Limited; 2023^-10Methocarbamol. Package Insert. Granules Pharmaceuticals Inc.; 2023

When used as directed, muscle relaxers can provide much-needed relief for several muscle-related conditions. With responsible use and careful monitoring, the benefits of muscle relaxers may be maximized while minimizing potential harm.