L-citrulline is an amino acid found in watermelon and other melons that improves blood flow. It does this by creating nitric oxide, an essential molecule that widens blood vessels. As a result, it improves circulation and benefits many ailments, including heart disease and clogged arteries. In addition, in healthy people, it boosts athletic performance and speeds recovery. Its effect on blood vessels improves blood flow to the penis, which enhances erectile rigidity and prevents erectile dysfunction (ED).