Muscle spasms can be caused by musculoskeletal conditions, such as back pain and neck pain, or neurological conditions associated with upper motor neuron lesions, such as multiple sclerosis. Both underlying causes can result in localized tenderness, significant pain, limitation of movement, and disability.

The drug class of skeletal muscle relaxants is a diverse group of medications that have been approved by the U.S. Food and Drug Administration (FDA) for either treatment of spasticity caused by neurological disorders (baclofen, dantrolene, and tizanidine) or for treatment of musculoskeletal conditions (carisoprodol, chlorzoxazone, cyclobenzaprine, metaxalone, methocarbamol, and orphenadrine). Many different classes of drugs, from antidepressants to benzodiazepines to antiepileptics, are also used to treat painful muscle spasms, but are not classified as skeletal muscle relaxants or approved for this indication by the FDA.

The "Drug Class Review on Skeletal Muscle Relaxants" compares the safety and effectiveness of nine drugs. A summary of the findings is below.

How do muscle relaxants compare in treating spasticity caused by a neurological disorder?

Baclofen, dantrolene, and tizanidine are effective treatments for patients with spasticity.

When compared to each other, tizanidine and baclofen are similar in their ability to reduce muscle spasms and improve function. There is insufficient comparative evidence both for dantrolene and for the skeletal muscle relaxants which have not been FDA-approved to treat spasticity. [full review]

How do muscle relaxants compare in treating muscle spasms caused by musculoskeletal conditions?

In patients with acute back pain or neck pain, cyclobenzaprine is consistently more effective than placebo for relieving pain, treating muscle spasms, and increasing function. There is also fair evidence that cyclobenzaprine is similar, and possibly superior, to diazepam for treating patients with musculoskeletal conditions.

However, cyclobenzaprine is not clearly effective in patients with fibromyalgia - although patients were more likely to self-report an improvement, there were no clear differences for measures of sleep quality, pain relief, fatigue, and tender points.

Of the remaining skeletal muscle relaxants, only tizanidine, carisoprodol, and orphenadrine have been shown to be more effective than placebo in the treatment of muscle spasms. [full review]

How do muscle relaxants compare in adverse effects?

In patients with spasticity, the overall tolerability of skeletal muscle relaxants seems to be similar, although different medications may be associated with different adverse effects. For example, when tizanidine was compared to baclofen, more patients on tizanidine experienced dry mouth while more patients on baclofen experienced weakness.

In patients with muscle spasms caused by musculoskeletal conditions, there is not enough evidence to judge whether one skeletal relaxant is safer than another.

In all patients, serious side effects with skeletal muscle relaxatns appear rare, but include potentially fatal liver damage with dantrolene. Tizanidine appears to be associated with asymptomatic, reversible elevations of aminotransferases, and both tizanidine and chlorzoxazone have been associated with rare cases of serious hepatotoxicity. There is no data for the comparative risk of abuse and addiction from skeletal muscle relaxants, although there are numerous case reports, almost all of which are associated with carisoprodol, a drug which is metabolized to meprobamate, a federally controlled substance. [full review]

Does age, gender, or ethnicity influence the safety or effectiveness of skeletal muscle relaxants?

There is almost no data that compares the safety or effectiveness of skeletal muscle relaxants in patients of different age groups, gender, or race. [full review]

Drugs included in this review

Further information

This PubMed Clinical Q&A was reviewed by Roger Chou, MD.

For the full report and evidence tables, please see:
Chou R, Peterson K. Drug Class Review: Skeletal Muscle Relaxants: Final Report [Internet]. Portland (OR): Oregon Health & Science University; 2005 May. Available at: http://www.ncbi.nlm.nih.gov/books/NBK10690/.