Taking L-citrulline supplements gives you more arginine in your blood than taking the same amount of L-arginine, because your body doesn’t break down L-citrulline as much when it’s digested, and taking 3 grams twice a day works better than the same dose of L-arginine.

Taking L-citrulline by mouth raises your blood levels of L-arginine better than taking L-arginine directly, because your body processes L-citrulline more efficiently and doesn’t break it down too early in the liver.

Taking a specific amino acid called L-citrulline twice a day for a week boosts a related chemical in your blood more than taking that chemical directly, which might help your body make more of a molecule that relaxes your blood vessels.

Taking a specific amino acid called L-citrulline twice a day for a week helps raise levels of another amino acid, L-arginine, in the blood more than taking L-arginine directly—this might help your body make more nitric oxide, which is good for blood flow.

Taking L-arginine pills in these amounts doesn’t raise the levels of L-arginine or related molecules in your blood or urine, because your body breaks them down too quickly before they can get into your system.

Taking a specific supplement called L-citrulline every day for a week, plus a little more before working out, can boost a key chemical in your blood by about three-quarters — and that chemical helps your body make nitric oxide, which is good for blood flow.