Quick answer: Both amino acids feed nitric oxide. Arginine is used directly but much of it is broken down before reaching the blood. Citrulline is turned into arginine by the kidneys and raises levels more steadily. Trials of both at gram doses found small blood pressure drops.
L-arginine
Arginine is the direct raw material for nitric oxide. A meta-analysis of trials using 4 to 24 g a day found an average fall of about 5 mmHg in systolic pressure. MedlinePlus on L-arginine.
L-citrulline
Citrulline is found in watermelon. It skips most of the breakdown in the gut and liver, then becomes arginine in the kidneys. Trials at around 3 to 6 g a day found modest effects.
Why combine them
Citrulline gives a slower, steadier rise; arginine a quicker one. That is the logic behind many combined formulas.
Cautions
Both can add to blood pressure medicines. Arginine should not be used after a recent heart attack and may trigger herpes outbreaks.
Read Melonex and blood pressure medicine.
More Melonex guides
General information, not medical advice. Melonex is a dietary supplement and is not intended to diagnose, treat, cure or prevent any disease.