Garlic has been suggested to lower blood pressure, but scientific studies have provided conflicting results. A study conducted at the Hartford Hospital, USA, examined the effect of garlic on blood pressure in patients with and without elevated systolic blood pressure with a meta-analysis of ten randomized controlled trials . They found that garlic reduced systolic blood pressure by 16.3 mm Hg and diastolic blood pressure by 9.3 mm Hg in patients with elevated systolic blood pressure. There is no effect of garlic in patients without elevated systolic blood pressure.
Sun X and Ku DD at the University of Alabama, USA, demonstrated a protective effect of allicin against the development of right ventricular pressure and right ventricular hypertrophy in monocrotaline-induced pulmonary hypertensive rats . They fed rats with a diet supplemented with 1% raw garlic before and after monocrotaline injection and found that the raw garlic feeding significantly inhibited the development of right ventricular pressure and right ventricular hypertrophy. Similar experiments with either boiled garlic or aged garlic, which do not contain allicin, were ineffective.
A study by the Sheba Medical Center, Israel, showed that the protective role of allicin against atherosclerosis, is not only the direct result of its antioxidant activity but also of other mechanisms, such as lipoprotein modification, inhibition of LDL uptake and degradation by macrophages . The researchers found that dietary supplementation pure allicin significantly reduced the atherosclerotic plaque area in apolipoprotein E-deficient and low-density lipoprotein receptor knockout mice.
 Effects of garlic on blood pressure in patients with and without systolic hypertension: a meta-analysis. Ann Pharmacother. 2008 Dec;42(12):1766-71
 Allicin in garlic protects against coronary endothelial dysfunction and right heart hypertrophy in pulmonary hypertensive rats. Am J Physiol Heart Circ Physiol. 2006 Nov;291(5):H2431-8.
 The antiatherogenic effect of allicin: possible mode of action. Pathobiology. 2005;72(6):325-34.