Ornithine is a chemical called an amino acid. It is made in the body. It can also be made in a laboratory. People use it as a medicine. Ornithine is commonly used by mouth for improving athletic performance. It is also used for weight loss, wound healing, and to increase sleep quality. But there is limited scientific research to support these other uses. It’s not known how ornithine might work for medical uses. However, ornithine might help to increase levels of the amino acid, arginine. It might also increase levels of hormones that increase muscle size. Athletic performance. Taking ornithine by mouth might reduce fatigue and improve measures of athletic performance such as speed, strength, and power in some people. Taking ornithine in combination with arginine also seems to improve strength and power in male weightlifters.