Carisoprodol (Soma®) versus Cyclobenzaprine (Flexeril®)

Based on "Essential Pain Pharmacology"
written by Howard S. Smith, MD; Marco Pappagallo, MD

Muscle relaxants do not share many similarities other than their intended purpose. This makes it difficult to identify any one as the “best muscle relaxant”.

Cyclobenzaprine advantages over Carisoprodol

• Unlike carisoprodol, cyclobenzaprine is not addictive and has a non-controlled status.
• Suitable for patients with addiction or drug misuse issues.
• Available in once-daily formulation under the trade name Amrix®.
• The sedative properties of cyclobenzaprine may benefit patients with seep problems.
• FDA pregnancy category B.
• Cyclobenzaprine has an opioid-sparing effect when used concurrently with painkillers such as codeine or hydrocodone.

Carisoprodol advantages over Cyclobenzaprine

• Carisoprodol is not contraindicated in patients with glaucoma, arrhythmias, recent myocardial infarction, or congestive heart failure.
• Carisoprodol is a very potent and effective muscle relaxant.
• Carisoprodol has no considerable anticholinergic activity.

Carisoprodol	Cyclobenzaprine
Brand names
• Soma ® • Sanoma ® • Carisoma®	• Flexeril ® • Amrix ®
Drug class
Skeletal muscle relaxant, centrally acting
Dose formulations
• Tablets	• Tablets • Capsules, extended-release
Legal status
• Rx only • Schedule IV controlled substance	• Rx only • Not a controlled drug
FDA-approved indications
• Acute painful musculoskeletal conditions • Both medications alleviate acute musculoskeletal pain and increase mobility
"Off-label" uses
• Fibromyalgia	• Fibromyalgia • Sleep aid
Mechanism of action
• Carisoprodol produces muscle relaxation by blocking interneuronal activity in the descending reticular formation (neural network in the brainstem) and spinal cord. • Both carisoprodol and its active metabolite meprobamate produce barbiturate-like effect at GABA-A receptors.	• Cyclobenzaprine acts mainly at the brainstem level to decrease tonic muscle activity. It influences both the alpha and gamma motor neurons. Also, cyclobenzaprine has activity at spinal cord level at some extent.
• No specific antinociceptive properties.
Onset of action
• 30 minutes	• within 1 hour
Duration of action
• 4 to 6 hours	• 12 to 24 hours
Half-life
• Carisoprodol: 2 hours • Meprobamate: 10 hours	• ~18 hours (range from 8 hours to 3 days)
Metabolism, Elimination
• Carisoprodol is metabolized by liver enzymes CYP2C19, and is extensively converted to meprobamate.	• Cyclobenzaprine is extensively metabolized by the liver via glucoronide conjugation and N-demethylation. • It is primarily eliminated by the kidneys. • Excreted in urine, feces.
Contraindications
• History of acute intermittent porphyria • Hypersensitivity to carisoprodol or meprobamate	• Hypersensitivity to cyclobenzaprine • Hyperthyroidism • Cardiac arrhythmias • Conduction disturbances • Congestive heart failure • Acute recovery phase of myocardial infarction • Concomitant use with MAO inhibitors - risk of life-threatening interactions with MAO inhibitors
Side effects
• Drowsiness • Dizziness • Headache • Orthostatic hypotension • Syncope • Tachycardia • Agitation • Irritability • Depression • CNS respiratory depression • Seizures • Withdrawal symptoms • Drug abuse and dependence. Substance abuse is a serious problem with carisoprodol, probably as a result of meprobamate formation.	• Drowsiness • Dry mouth • Dizziness • Headache • Fatigue • Palpitations • Bad taste in mouth • Indigestion • Blurred vision • Constipation • Increased intraocular pressure
Drug interactions
• Potential for drug interactions with CYP2C19 inhibitors and inducers	• Dangerous interactions with MAO inhibitors
Pregnancy category
C	B

Carisoprodol vs Cyclobenzaprine for back pain and spasms

Head-to-head comparative trial1 in patients with acute back pain and spasms demonstrated that cyclobenzaprine and carisoprodol provide similar improvement in:

• pain
• muscle stiffness
• activity impairment
• sleep impairment
• tension

In this study dry mouth was more frequent with cyclobenzaprine (38% vs. 10%) and dizziness less frequent (8% vs. 26%). Withdrawal rates due to adverse effects were equal (8%) 1.

Further reading

• Carisoprodol (Soma) Facts
• Cyclobenzaprine vs Methocarbamol

References

• 1. Rollings HE, Glassman JM, Soyka JP. Management of acute musculoskeletal conditions - Thoracolumbar strain or sprain: A double-blind evaluation comparing the efficacy and safety of carisoprodol with cyclobenzaprine hydrochloride. Curr Ther Res. 1983;34(6):917-928.

Published: March 31, 2008
Last reviewed: October 30, 2017