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 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 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 a 5-gram dose of L-arginine won’t help healthy young men cycle better or lower ammonia in their blood, even though it does raise levels of a few other related chemicals in their blood.