According to the American Heart Association, there is no formal definition for low blood pressure. Normal blood pressure is 120 over 80 millimeters mercury or less. Unless unpleasant symptoms occur, the lower the blood pressure, the better.
The American Heart Association says that chronically low blood pressure readings are not of concern unless symptoms such as fainting, dizziness and blurred vision occur. Low blood pressure commonly occurs during the first few weeks of pregnancy, after bed rest and in response to certain medications and other conditions. Sudden drops in blood pressure may signal the presence of more alarming conditions such as dehydration, sudden blood loss and septic shock.