Carnitine

Carnitine is an amino acid. It is an ammonium compound used in the metabolism of most mammals, plants, and some bacteria. 
When getting energy (ATP) from nutrients, carnitine transports long-chain fatty acids into mitochondria so that it can be oxidized for energy production. Carnitine also also takes part in removing products of metabolism from cells. Because of its important role, carnitine is concentrated in tissues like skeletal muscle and cardiac muscle that metabolize fatty acids as an energy source.
Doping in sport.
L-Carnitine is not on the doping list of World Anti-Doping Agency, or WADA, but people in sport got punished in 2019, for the way they used L-Carnitine. (See doping in sport.)