Tricaine mesylate (Tricaine methanesulfonate, TMS, MS-222), is white powder used for anesthesia, sedation, or euthanasia of fishes (mainly salmonids). TMS is the only anesthetic licensed in the United States for fin fish that is intended for human consumption. TMS is a muscle relaxant that operates by preventing action potentials. By blocking action potentials, no signals can be exchanged between the brain and the extremities. There will be no sensory input or muscle contractions which would have been caused by action potential, which includes most muscles. The optimum concentration used is 50-75 ppm (parts per million). However, the optimum may vary with the size and species of the fish, and other variables.