No side effects of L-citrulline are currently known, as of 2015. However, there is not enough scientific evidence to know if it is safe for women who are pregnant or nursing, according to WebMD.
L-citrulline is an amino acid that occurs naturally in foods such as watermelon and is also produced by the body, explains WebMD. It is used to help treat Alzheimer's disease, dementia, fatigue and muscle weakness. Sickle cell disease, erectile dysfunction, high blood pressure and diabetes are also treated with L-citrulline. Additionally, some people use it for body building, increasing energy and athletic performance. However, there is not enough evidence to fully support these claims. In one study with a treadmill, people who took L-citrulline actually became fatigued faster than those who did not.
The body uses L-citrulline by turning it into L-arginine, another amino acid, and nitric oxide. L-citrulline may help increase the body's supply for making certain proteins and open up the veins to improve blood flow and decrease blood pressure, notes WebMD. A proper dose of L-citrulline depends on the user's age, health and various other conditions. There is also not enough information available to determine the correct dose. It is recommended to follow the directions on the supplement label.