Garlic supplements have shown effectiveness in reducing blood pressure. It contains allicin, a substance that lowers blood pressure and relaxes blood vessels. Collectively, these factors may help lower blood pressure. In a study with high blood pressure found that taking garlic reduced systolic and diastolic blood pressure by an average of 8.3 mm Hg and 5.5 mm Hg, respectively. This reduction was similar to the effects of blood pressure medications.