The aim of this work was to evaluate in a double-blind controlled study the effect of a pure muscle relaxant drug (dantrolene sodium) in the symptomatic treatment of uncomplicated acute low back pain in 20 patients. The one capsule daily dosage (25 mg) was given for 4 days. Patients treated with dantrolene sodium obtained an improvement of muscle contracture (p < 0.001 versus placebo). VAS pain measurement, and pain behavior (both p < 0.001 versus placebo). The electromyogram (EMG) parameters (i.e., the antalgic reflex motor unit firing) improved in the treated group (p < 0.01 versus placebo). Data show the possibility of treating uncomplicated acute low back pain with a pure muscle relaxant. Some basic physiopathological aspects of low back pain are also discussed.