L-Citrulline is an over-the-counter amino acid that also occurs in foods such as watermelon. It is often used as a supplement in various situations, for example in sports.

No, it is classified as a non-essential amino acid. This means it does not necessarily have to be consumed through food, but supplementation is still possible.

Essential amino acids must be obtained through diet because animal organisms cannot produce them themselves. Non-essential amino acids can be produced by the organism itself but are sometimes also taken additionally through supplements.

There are 20 standard amino acids considered the building blocks of proteins. Of these, 9 are essential; the rest are non-essential or, under certain circumstances, semi-essential.