This is a double-blind, randomized, placebo-controlled, multicenter study of the efficacy and safety of tolperisone (a non-opioid) or placebo administered in subjects with pain due to acute back muscle spasm.

Inclusion Criteria:

• Ambulatory
• Current acute back pain due to acute muscle spasm starting within 7 days prior to study entry (Day 1) and at least 8 weeks following resolution of the last episode of acute back pain.
• Willingness to discontinue all previous or ongoing treatment of pain or muscle spasm on study entry at Day 1 through the end of treatment including medication, acupuncture, chiropractic adjustment, massage, transcutaneous electrical nerve stimulation [TENS], or physiotherapy.
• Pain must be localized from the neck (C-3 or lower) to the inferior gluteal folds and spasm assessed during the Screening physical examination.
• Body mass index range between 18 and 35 kg/m², inclusive.

Exclusion Criteria:

• Presence of acute or chronic back pain for the previous 8 days or longer, where back pain is present on more days than not.
• Presence of neurogenic pain in the back, neck, upper or lower extremities, including pain from (or suspected from) nerve root compression or injury (radicular pain or "pinched nerve") or neuropathic pain. Evidence of these types of exclusionary pain includes radiation of pain that radiates beyond the back, chronic pain, and pain associated with abnormal sensation or loss of sensation in the back or extremities.
• Presence of pain anywhere other than the target back pain that is bothersome, interferes with activity, or for which pain relief is taken.
• History of any neck, back, or pelvic surgery.
• History within the previous 3 years of: spinal fracture or spinal infection; inflammatory arthritis; degenerative spine disease; or any other back or spine condition that may reasonably contribute to current back pain.
• Subjects who test positive for alcohol by breathalyzer test or have a positive urine drug screen for drugs of abuse (e.g. amphetamines, methamphetamines, barbiturates, benzodiazepines, cocaine, cannabinoids, oxycodone, opiates), including cannabis even where legal, at Screening.