Optimal blood pressure is less than 120/80 (systolic pressure is 120 AND diastolic pressure is less than 80). Within certain limits, the lower your blood pressure, the better. There is no specific number at which day-to-day blood pressure is considered too low, as long as no symptoms of trouble are present. Most doctors consider chronically low blood pressure dangerous only if it causes noticeable signs and symptoms, such as dizziness or lightheadedness, fainting, dehydration, lack of concentration and blurred vision. Severely low blood pressure can be an indication of a serious heart, endocrine or neurological disorder.