This stable-labeled internal standard is suitable for quantitation of tizanidine levels in GC/MS or LC/MS methods for clinical toxicology, urine drug testing, pain prescription monitoring, forensic analysis or isotope dilution applications. Tizanidine, sold under the tradename Zanaflex^®, is a skeletal muscle relaxant used for treatment of spasticity associated with multiple sclerosis, and for painful muscle spasms.