Metaxalone-d6 is intended for use as an internal standard for the quantification of metaxalone by GC-or LC-MS. Metaxalone is a skeletal muscle relaxant. It inhibits the proliferation of, and induces apoptosis in, RAW 264.7 cells in vitro when used at concentrations ranging from 1 to 100 µM. Metaxalone also reduces LPS-induced increases in COX-1, COX-2, and NF-kB levels and inhibits LPS-induced production of TNF-α, IL-6, and prostaglandin E2 in RAW 264.7 cells. Formulations containing metaxalone have been used in the treatment of lower back pain.