Tolperisone in Acute Muscle Spasm of the Back

Inclusion Criteria: * Ambulatory * Current acute back pain and/or stiffness due to acute and painful muscle spasm starting within 7 days prior to study entry and more than 8 weeks after the last episode of acute back pain. * Pain of 4 or more on the subject "right now" rating of pain intensity Numeric Rating Scale (NRS) scale of 0-10. * Willing to discontinue all medication used for the treatment of pain or muscle spasm on study entry. * Pain localized below the neck and above the inferior gluteal folds. * Body mass index ranging between 18 and 35 kg/m². Exclusion Criteria: * Unwillingness to stop taking pain or antispasmodic medication other than the study medication (specifically opioid use \[e.g., Vicodin\], barbiturates, and cannabis). * Chronic pain for the previous 3 months or longer, on more days than not. * Radicular pain in the lower extremity (i.e. pain radiating below the knee), sciatica pain down the leg, or pain below the knee (indicating a lumber radiculopathy); radicular pain in the upper extremity, radiating into the forearm or hand (indicating a cervical radiculopathy). * Concomitant severe pain in a region other than the back. * Spinal surgery within 1 year of study entry. * Back pain due to major trauma (e.g., motor vehicle accident, fracture of bone) unless resolved for more than 1 year. * Treatment of back pain ongoing with non-pharmacological therapy (e.g., acupuncture, chiropractic adjustment, massage, Transcutaneous Electrical Nerve Stimulation \[TENS\], physiotherapy). * Subjects who test positive for alcohol by breathalyzer test. * Unwilling to stop taking moderate to potent inhibitors of cytochrome P450 (CYP) isozymes CYP2D6 and CYP2C19, which are likely to cause drug interactions with tolperisone HCl (e.g., medications such as paroxetine and fluvoxamine).