L-citrulline is a nonessential amino acid, meaning the body already makes it naturally. Glutamine, which is the most abundant amino acid in the body, mainly produces L-citrulline. Unlike many amino acids you may have heard of, L-citrulline is not used to make proteins. Instead, it has other functions in the body.

L-citrulline is a non-essential amino acid that helps the body produce nitric oxide and improve blood flow. Sold as a dietary supplement and a separate pharmaceutical version, L-citrulline is used to treat a rare genetic disorder and may help with certain vascular problems and other medical conditions.