Ornithine is a non-proteinogenic amino acid that plays a role in the urea cycle. Ornithine is abnormally accumulated in the body in ornithine transcarbamylase deficiency. The radical is ornithyl.

l-Ornithine is one of the products of the action of the enzyme arginase on l-arginine, creating urea. Therefore, ornithine is a central part of the urea cycle, which allows for the disposal of excess nitrogen.

l-Ornithine supplementation attenuated fatigue in subjects in a placebo-controlled study using a cycle ergometer. The results suggested that l-ornithine has an antifatigue effect in increasing the efficiency of energy consumption and promoting the excretion of ammonia.

Ornithine is used for improving athletic performance, reducing glutamine poisoning in the treatment of a brain condition due to liver disease (hepatic encephalopathy), and for wound healing.