This study is testing if a new version of a medicine called metaxalone 640 mg can reduce pain in your lower back and legs. The study is double-blind, meaning neither you nor the doctors know if you're getting the real medicine or a look-alike pill with no medicine (placebo). It's also randomized, so everyone has an equal chance of getting the real medicine or placebo. This trial includes people aged 18 to 80 who have new back or leg pain. Participants will answer questions about their pain, physical activity, and sleep on Day 1 and again on Day 7 after treatment. The main goal is to see if the new medicine helps more than the placebo.

• The study lasts 7 days with surveys on Day 1 and Day 7.
• You must have a pain level of 6 or higher on a pain scale to join.
• If you're taking muscle relaxants, pain relievers, or certain other medications, you can't join.