Muscle relaxants are used to reduce muscle spasms that can be caused by a number of conditions that affect the nervous system. Conditions that can cause muscle spasms include multiple sclerosis, motor neurone disease, and cerebral palsy. Muscle spasms and strains can also lead to long-term head or back injuries. Muscle spasms can also occur as part of a shorter-term condition or injury, such as low back pain or whiplash. Medicines that help the muscles relax can also relieve pain and discomfort.