Doctors at the University of Medical Science in Poznan, Poland randomized patients with high blood pressure to either L-arginine (2 or 4 g three times daily or placebo. Blood pressure (both systolic and diastolic) showed significant lowering after 4 weeks of L-arginine supplementation only in the subgroup of patients treated with 12 g of L-arginine daily.
Comments: Although this amino acid appears to be helpful in high dosages, it may be expensive to take this large amount and to remember taking it three times daily. There are other natural options to treat hypertension, see http://www.raysahelian.com/hypertension.html