Most experts define low blood pressure as a reading lower than 90 for systolic pressure and lower than 60 for diastolic pressure, according to Mayo Clinic. Only one of the numbers needs to be in the low range for the overall blood pressure reading to be considered low.
As long as a patient is not exhibiting worrisome symptoms, such as dizziness, dehydration, nausea, fainting, fatigue and depression, there is no particular blood pressure reading that is considered too low, notes the American Heart Association. Low blood pressure can occur with a variety of conditions, including pregnancy, heart or endocrine problems, allergic reaction, nutritional deficiencies and decreases in blood volume. Certain medications, such as diuretics, narcotics and antidepressants, may also cause low blood pressure. A sudden drop in blood pressure can cause dizziness and fainting and may be life threatening in some cases, according to the Mayo Clinic.