Taking L-citrulline or L-arginine pills for eight days doesn’t boost the body’s nitric oxide levels in elite swimmers and triathletes during their normal training, so these supplements don’t seem to help improve blood flow in this situation.

Taking L-citrulline or L-arginine supplements for eight days doesn’t make swimmers any faster in 100m or 200m races, even if they’re trained athletes.

Taking L-citrulline or L-arginine supplements doesn't seem to help competitive swimmers swim faster in 100m or 200m races, even though scientists thought these supplements might help by improving blood flow or reducing muscle burn.

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.

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 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.