Skeletal muscle relaxants are drugs that are used to relax and reduce tension in muscles. They are more simply referred to as muscle relaxants.

Some work in the brain or spinal cord to block or dampen down excessively stimulated nerve pathways. These are called centrally acting muscle relaxants and examples include baclofen, methocarbamol, and tizanidine.

Others act directly on muscle fibers and are classified as peripherally acting muscle relaxants. Examples include dantrolene and the different types of botulinum toxin. Although dantrolene acts directly on the muscle itself, it also appears to indirectly act on the central nervous system and can cause drowsiness.

Cannabis extract also has muscle relaxing properties and is thought to act both centrally and peripherally.

What are skeletal muscle relaxants used for?

Skeletal muscle relaxants are mainly used to treat:

• spasticity, which is another term for stiff and rigid muscles caused by conditions such as cerebral palsy, multiple sclerosis, or stroke
• muscle spasms which are temporary muscular contractions that are often associated conditions such as tension headache, low back pain, or fibromyalgia
• cervical dystonia - a painful condition where the neck muscles involuntarily contract, causing your head to uncontrollably tilt forward or backward.

What are the differences between skeletal muscle relaxants?

Skeletal muscle relaxants differ in the way they work (centrally or peripherally as discussed above), their side effects, and their effectiveness for certain conditions.

Note that several other medicines, notably diazepam, may also be used as a muscle relaxant and are not listed below.

Are skeletal muscle relaxants safe?

Evidence supporting the effectiveness of skeletal muscle relaxants for muscle spasm is sparse; most trials are old and not of good quality. Skeletal muscle relaxants consist of a varied range of medicines and some may not be suitable for people with certain medical conditions such as an enlarged prostate, epilepsy, glaucoma, intestinal problems, liver or kidney disease, or myasthenia gravis. Many also interact with other medications.

Some, like dantrolene, can adversely affect the liver and blood samples should be taken before treatment to check for any pre-existing liver disease or to establish how well the liver is functioning before treatment, and what effect the drug subsequently has.

Muscle relaxants can affect overall muscle tone and may be dangerous if muscle tone is needed for safe balance or movement. Alcohol can enhance these effects. Many muscle relaxants need to be tapered off slowly, rather than abruptly stopped.

What are the side effects of skeletal muscle relaxants?

Drowsiness is common, particularly with centrally acting muscle relaxants; however, drowsiness can occur with some peripherally acting muscle relaxants, such as dantrolene, as well. This may impair the ability of a person to drive or operate machinery or perform hazardous tasks.

Some may also cause side effects such as a dry mouth, fast heartbeat, gastrointestinal upset (such as nausea, vomiting, constipation), headache, insomnia, light headedness, problems with urination, and other unwanted effects.

Products containing botulinum toxin may cause generalized muscle weakness, vision changes, breathing difficulties and other serious side effects if the toxin spreads from the area of injection.