Hey, magnesium CAN lower BP further  however a lot of foods and supplements can lower BP and the benefits of magnesium are phenomenal so you most likely may still benefit. if you are to take it, check with your doctor first. Secondly, take them in the evening or before be when you are less active. thirdly do a lower dose. Also magnesium can help improve and strengthen the cardiovascular system which can overtime actually improve BP in the long run.