Mephenesin-d 3 is deuterium labeled Mephenesin. Mephenesin is an NMDA receptor antagonist. Mephenesin is also a central muscle relaxant with antianxiety, muscle-paralyzing and anticonvulsant effects. Mephenesin acts directly on the skeletal muscle fibres to produce skeletal muscle relaxation. Mephenesin is promising for research of spasticity or painful muscle spasm.