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.

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 can raise the level of arginine in your blood, but that doesn’t always mean you’ll produce more nitric oxide or get better at exercising—sometimes it helps, sometimes it doesn’t.

Taking a specific amino acid supplement called L-citrulline twice a day for a week helps healthy people with high levels of a certain compound in their blood to produce more of a helpful molecule that improves blood flow.

Taking a specific amount of arginine supplement for just three days doesn’t change the levels of nitric oxide in the blood of highly trained male athletes when they do short, intense workouts.