L-Citrulline is a non-essential amino acid. The kidneys change L-Citrulline into another amino acid called L-Arginine and a chemical called nitric oxide. These compounds are important to the heart and blood vessel health. They may also boost the immune system.
So L-Citrulline works similarly to L-Arginine, i.e. may help with erection quality. However, there are indications that L-Citrulline supplementation is a more effective method of increasing the L-Arginine level in the body then L-Arginine supplementation. This is because L-Citrulline turns into L-Arginine in the kidneys after supplementation.