L-arginine is an amino acid naturally found in red meat, poultry, fish, and dairy. It is necessary for making proteins and is commonly used for circulation.
Ornithine is an amino acid that is made in the body. It's not used to create protein, but plays a role in other processes. It can also be made in a lab.
L-arginine is converted in the body into a chemical called nitric oxide. Nitric oxide causes blood vessels to open wider for improved blood flow. L-arginine also stimulates the release of growth hormone, insulin, and other substances in the body. It can be made in a lab and used in supplements.
Ornithine might help to increase levels of another amino acid called arginine. It might also increase levels of hormones that increase muscle size.
People use L-arginine for chest pain and various blow flow issues, erectile dysfunction, high blood pressure during pregnancy, and a serious disease in premature infants called necrotizing enterocolitis (NEC). It's also used for many other conditions.
People use ornithine for athletic performance, dry skin, insomnia, wound healing, and other purposes. Don't confuse ornithine with ornithine alpha-ketoglutarate (OKG) or L-Ornithine-L-Aspartate. These are not the same.